use 3g phone (japan) what kind of 3g sim card should i buy ?

I have done some simple research. It is funny that all six mobile networks' roaming charges for Japan are almost the same. So there is almost no difference among each other.
One thing you have to know is that if you are using roaming, the quality would depend on the roaming partner but not the host operator in Hong Kong. As a matter of fact, all six mobile network operators in Hong Kong use Softbank and Docomo as roaming partners in Japan. So, quality-wise, there will be no difference at all.
Perhaps the only difference would be the quality of customer services. CSL is the best in Hong Kong but the most expensive. However, if you use this card only for roaming, then you shall opt CSL since the roaming fees for Japan are similar among six MNOs.

2007-07-03 12:43:12 補充：
While there is no such a thing called 3G SIM, there are ony 3G phones. 2G and 3G are using the same SIM. So, even SIM issued by non-3G opeators like People can also roam in Japan if you are carrying a 3G phone (WCDMA).