To measure the microtremor of the ground, a 50 meter modified Fabry-Perotinterferometer has been installed in an underground tunnel in the Nagatsuta Campus of the Tokyo Institute of Technology. It is illuminated by a thermally stabilized single mode 633nm He-Ne laser. Only one of the axial modes is excited by using three mode-matching lenses. The length of the interferometer is controlled to satisfy its resonance condition by using a piezoelectric translator (PZT) mounting a reflection mirror. The applied voltage to the PZT is almost proportional to the amplitude of the microtremor. The conversion accuracy is about 2x10^-10.