I don't know if Tokyo or Japan has rental stores. If so, you might suggest to the nearest one that they buy a good surplus military detector or two (with batteries) and rent them out. I'll bet there are LOTS of people concerned enough to rent one for a day or two just to reassure themselves. Someone could make a TON of money!
There are simple radiacmeters, modern ones, oldfashioned ones that work as well as they ever did. (My US Army NBC Training was back in '62, and I stayed active in it through '65 or so, but I'm definitely out of date. But we had old
Here's a simple gamma dose radiacmeter, very easy to use, simple, bulletproof, $160 US:
http://www.majorsurplus.com/Radiacmeter-IM-179U-Military-Gamma-Dose-Rate-Meter-Issued-Certified-P14342.aspx
Here's the big old IM-174A we used back in The Day for gamma radiation surveys. Some of them might still be around in surplus sales:
http://www.civildefensemuseum.com/southrad/im174apd-1.html
And the old AN/PDR-27. This one's US Navy, but just like the Army one I trained on. Measures alpha, beta, gamma:
http://www.ecrater.com/p/11844783/cr071-geiger-counter-radiacmeter-navy
The problem with the military stuff is finding the appropriate batteries, or some sort of conversion kit to use modern batteries.
There are civilian meters, of course, that you or your rental shop could buy new. It makes far more sense for a rental shop to do this sort of investment though. Not every civilian needs constant or regular monitoring: just the occasional sampling for peace of mind as you describe.
One thing: if you decide to clean out that drain, use rubber gloves, don't raise dust you might breath (or use a dust mask), wash everything outside (so potentially contaminated mud doesn't collect in your inside drains, etc. It's the alpha particle emitters that can really cause problems if they get inside your body.