PURPOSE: To facilitate handling without having various restrictions and to obtain a stable thickness signal by using a microwave and processing two mutually 90° out-of-phase interference signals, and measuring the thickness of a sheet type running body.

CONSTITUTION: A sensor part 1A has a microwave Doppler sensor 2A, phase shifter 3A, transmitting/receiving antenna 4A, etc., and the antenna 4A is installed facing one surface of the sheet type body 20. The sensor 2A has a Gunn diode 5A, power source 6A, and crystal detector 7A, and transmits the microwave toward the antenna 4A and also detects 9 part of the transmit signal and a receive signal by the detector 7A. Sample holding circuits 11A and 12A sample and output the signal detected by the detector 7A. A switching circuit generator 13A generates a signal for driving a pin diode 9A, the circuits 11A and 12A, etc.