There are some small, furnished serviced apartments owned by Japanese companies who specialize in such things and have them throughout Japan.
They are not a bad option for people who are only in Japan for 6 months or a year because we don't need to buy furniture and so on.
Be careful though, as they usually require about a months rent as "key money" which is not refundable (standard in Japan), as well as a substantial bond.
Despite at least one of them advertising on the net that a Japanese garantor is not required - that is often a misrepresentation too and one is often required (unless the place will be rented by your employer - which then ties you to that employer!)