Koukaku Fudou Senki Robo-san.

There was once a high-tech robot which saved the earth from aliens. People have long forgotten how to operate this giant machine and it has been placed in an isolated ghost town. Professor Tachibana believes in the mobility of this machine and won't give up in finding a suitable pilot for the robot. However, one day, he finds out that the robot became the gathering space for eight middle school girls!