IME, when they say "single bed room," they mean it has a "single bed." That is, a twin-sized bed that would be rather uncomfortable for two people. Many hotels also list "double bed rooms" which are more what you are looking for - they have one double bed. There are also some which list "semi-double beds" for one or two people. The "semi-double," which I prefer to book for myself, is wider than a twin but narrower than a double. It would be snug but doable for two small people, comfortable for one.

If you really don't care about Japanese vs. western hotels, you might try looking for the western "Comfort" hotel chain. They have quite a presence in some areas of Japan and feature western-style king-sized beds - a rarity in Japan.

JR