A plug converter and a transformer are two different things. The plug converter will simply act as an adaptor for your plug so that you can plug into the local power supply. However, if your appliance is built to handle a lower voltage than the local voltage, you could end up frying your appliance. Most electronics such as laptops, mobiles, PDAs etc. now have a transformer built in so they should be OK but you should check.
Japan uses a pretty low voltage (most places use around 200-240V) so you should be OK for Japan.
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Behold, I see my father and mother.
I see all my dead relatives seated.
I see my master seated in Paradise and Paradise is beautiful and green; with him are men and boy servants.
He calls me. Take me to him.