you may head to Dapsimni or Janhanpyeong - both are in purple subway line, station no #542 & #543 respectively. Once out of the station, you can see double barber poles spinning OR big barber poles along the roadside - which means sex massage...i presume u can't speak korean, just tell the "customer service manager" u want anma- he will then ask u which nationality u from, just mention sporean - which u hv to pay ard 100,000won to 120,000won...dun ever mention u are japanese...
service is for a hour or so...massage + sex. enjoy yr stay in seoul.
ps. if you hv loads of $ to spare, u can head to massage shop in gangnam which will set u back ard 180,000 to 200,000..the gals are more pretty and some shops come in theme class, etc classroom, army camp, jungle etc.