Please note all distance are taken from map measurements as my bike computer was broken at this time.
From Sopporo take the busy route 5 from near the central railway station north. After 2.8km route 5 heads west. By continuing north the road become route 231 which is the only road to follow for the whole day. After 9km route 231 swings east and just after this crosses a river over a fancy bridge. Just after the bridge route 231 turns off to the right and begins its journey up the coast.
Throughout this day there are times, which are apparent on the map where you can take a few shortcuts and get off the main road. Traffic is fairly light but there is quite a high proportion of trucks. All the towns along the coast have a Seicomart so there is no problem accessing food. According to the Touring Mapple map there are a number of camp sites as well as Youth Hostels, however, I never stopped to check out things like this.
The most memorable part of this section of road is the dramatic coast line with the hills running right to the water. Tunnels are used extensively to combat (I counted 18) this and many of them are quite old, narrow, and dimly lit so take care.
Rumoi is a large port town sitting at the junction of routes 231, 232 and 233. I got a little lost here trying to find somewhere to camp. Eventually I found myself on route 232 and heading north out of town. North of Runoi route 232 is the coastal road. When the road rejoins the coast there is a park between the road and the ocean which is suitable for camping (although I was asked if I was scared of the Russian sailors whose ships were in port). There are toilets and a Seicomart about 1km further north from the park if you need supplies.
Further north is a site that you won't be able to miss due to its great size. This section of Japan must be extremely windy (luckily not when I was there) as there are dozens of enormous electricity generating windmills strung out along the coast. The great scale of these machines can only really be seen if you are lucky as I was to see someone up on top of one doing repairs.
As you approach the town of Enbetsu a bridge must be crossed to enter the centre of town. Just before the bridge there are roads leading off to the right and left. The right road leads up to an expensive camping ground with loads of facilities. The road to the left leads to a large open parkland right on the beach with toilets, bbqs, and plenty of space to pitch a tent for the night. The sunset over the ocean was spectacular.
About 10km before Wakkanai there is a turnoff (still route 106 however) from the coastal road that climbs short and sweet to a ridge and the edge of Wakkanai, a port town. Stay on the main road and descend into town. If you are heading for the ferry terminal turn left just after crossing the railway line and follow the road around to the port authority and the ferrys that serve the islands of Rishiri and Rebun as well as providing a weekly service to the Russian island of Sakhalin during summer.
Check your guidebook for details of the ferrys. Bikes have an extra charge go in with the cars. Ssomeone will help you tie your bike to the railing and they even have wheel chocks for your wheels. If you have time it's worth stocking up at the supermarket close to the ferry terminal as food is quite expensive on Rishiri.
There is some great cycling and hiking to be had on the islands of Rishiri and Rebun but the weather really closed in on me so I didn't get the chance so I'll leave that to you to explore on your own.