You have two options:

1. Use your PC as is with an external 110-to-220V transformer.  Straightforward, but make sure the transformer's wattage is sufficient for your PC and that it's a high-quality model suitable for extended use.  (Cheap, underpowered ones have a disturbing tendency to catch fire if overloaded for long periods of time.)

2. Replace your PC's power supply (PSU) with a Japanese model.  Power supplies are cheap and fairly standardized, so this is pretty straightforward and shouldn't require anything more than a screwdriver, but do double-check the physical dimensions, output voltage and the type and number of output plugs to make sure the replacement is compatible.