reduce 120v ac supply to 100v without external transformer/device
There are two possible sources of difficulty. 

First, running a transformer at 20% higher than design voltage nominal runs a serious risk of driving the core into saturation if the mains happens to run a bit higher than nominal. However, part of Japan is 50Hz and part is 60Hz, and I am going to guess that your 120VAC is 60Hz, so chances are it is not a problem if the transformer is marked 50/60Hz. A 50 Hz 100VAC transformer can be run at 120VAC 60Hz. 

Secondly, the higher than normal output voltage could stress whatever it is connected to, such as capacitors. There could also be additional current that would cause the transformer to overheat. We have no way of guessing what might go on there, but chances are it is not good, 

You can buck the voltage with an autotransformer configuration (VA will be 1/5 of using a 120:100 step-down transformer), replace the transformer, or in some cases (eg. toroidal transformer) you may be able to add a few extra turns to buck the secondary voltage (presupposing the primary is okay, as discussed in point 1 above). Typical volts per turn is around 1 so it might take 5-10 turns of suitably heavy and insulated wire.  

Don't muck with this unless you are 110% sure you won't compromise the isolation between primary and secondary- that's a safety issue.

Frankly if this is piece of (say) music equipment that is being moved to different gigs and used by random people (for example) I would strongly suggest replacing the transformer with an approved unit and having the unit inspected for safety.