I was wondering what the laws are in Japan concerning FM Transmitters. I recently purchased one of eBay (http://www.ebay.com/itm/401047827071) I don't have a license or anything so I would like to know if I can legally use it.
Usually these type of questions are not easy to answer precisely.
Generally speaking countries allow certain low-power devices to transmit within the FM-broadcast band, pending type-approval of the device. (e.g. the device must be tested and found to be OK according to specifications)
There are resources which can be found on the internet, such as this, which lists the following:
As specified in the Radio Law, Extremely Weak (low) Power Equipment and Receivers must comply with the Japanese Radio Regulations, but do not require registration at MIC. Examples of products requiring performance of compliancy with the emission limits, are FM modulators and radio Receivers. Teleconformity offers Performance Certification, including testing.
You do need to ask yourself how much "authority" these resources have in these cases.
It seems that there is some sort of "certification" needed for low power devices, from MIC.
You could ofcourse contact MIC (Ministry of Internal Affairs and Communications) to get further information.
... maybe someone with more specific knowledge about Japanese legislation can give you a more precise answer...