If you move to Japan for a long period. No you cannot use your Malaysia Smartphone in Japan.

The only available prepaid simcard you can get in Japan is 'database' simcard which cause a lot 3k yen a month via b-mobile. (No Number, only data)
It will be very troublesome especially your colleague or friends try to contact you.

As long you have Japan Residence card, you can go to any official mobile provider store and apply for a new phone plan.
Normally you'll get between 3k-6k yen monthly with a smartphone for 2 years contract.
or like my friend, she directly purchase unlock iphone from softbank with standard 4k yen call/unlimited data plan with no contract. Basically she can cancel her plan anytime and bring back her unlock japan iphone back to Malaysia.

Any provider is fine in Japan just depends which plan you prefer.
Since my phone bill got covered by the company I work for.. I just pick Iphone unlimited data plan with number under Softbank Japan with 2 years contract. (between 3k-5500yen a month, once am reaching 500mb data they will automatically charge me 5500yen a month. or If my data didn't exceed 500mb, I only pay 3kyen a month).
Once am back in Malaysia, I just pay RM450 to unlock my Japan Iphone.

I know a few using NTT Docomo too...

the first one....totally didn't see the 3pin during my visit there

info: http://www.japan-guide.com/e/e2225.html

Well shit @_@

I thought CoE supposed to take a month or longer, not a single effing week, applied on 5th got today. Now I only have 2 week to prepare (the company wants me to come in on the 1st of August), apply visa, find temp housing, apply for international license, buy a suitcase, find a way to reject mimos job which I'm supposed to join next week *panik mode* I don't even have my todo list done yet.

oh you applying when? on its way as in via fedex?