Hasegawa san,
Did you remove your upper radiator hose anytime recently? If so, it has to go back on EXACTLY like it was before or else you might get a leak. In other words, don't automatically think that your radiator is leaking...it could be the hose. I experienced this recently myself. Evidently the old hose on there gets a crease in it where it went on so if it is moved any at all it might leak. Another possibiliy that I've experienced is that the hose from the over-flow (or expansion) tank that is not fitting well into the thermostat housing can leak and as the car backs up or goes down a hill, radiator fluid can move along the bottom of the large hose to the radiator and appear to be causing a leak. Check for radiator fluid or white residue from the dried up fluid along the bottom of the large hose as well as on or near the thermostat housing. If all of these check out then you may have a leak in the radiator itself. Usually any patching of the radiator is futile with things going progressively worse until you do have to replace it. If this is the case, then you can get a new one for around $200 from www.fcpgroton.com made by a company called Nissen. These are quite good but takes some effort getting all of the hoses off the old one and back on because your radiator has coolant, oil and transmission fluid running through it. Good luck, and if you happen to be a native Japanese, I'm bilingual and would be glad to correspond with you in your native language. Click on the envelope icon at the top of my message to send me an email.
MIJ
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.