Will Japan market gear work on US current?

IIRC, Japan uses 100v AC so you will probably need a transformer. But some gear has universal current adaptability.

The S2000's doesn't have universal adaptability (from what i
know) since there's a model for usa/uk and japan.

You need a step down transformer.

Japan is 100V AC. Most gear has universal voltage transformers though, which can be rewired for 120V operation. Sometimes there is even a switch to make the task easier.

Your best alternative to the Step down route, is to purchase a Buck/Boost transformer to 'buck' the 117/120 voltage down to 100 voltage. It has to be matched for wattage and amperage to the Yamaha unit. I use one for my Accuphase units. You could wire it yourself,(or ask an electrician) all come with instructions in the box
and the transformers can be found on Ebay all the time.