PROBLEM TO BE SOLVED: To minimize the disturbance of a signal and to enable the almost simultaneous evaluations of sensor elements by composing a transmission/ reception unit and the sensor element so that an inquiry signal and a reply signal may be time inverse with each other and the sensor elements may have different SAW propagation time. SOLUTION: In each of sensor elements 1, 2, 3,..., n, the converter 4 converting the electric signal received via an antenna 6 into a surface acoustic wave is included. This surface acoustic wave reaches a reflector 5 after preliminarily fixed propagation time and is reflected to be returned to the converter 4 from the reflector 5. The converter 4 converts this reflected and returned surface acoustic wave into the electric signal again. A transmission/reception unit 10 and the sensor elements 1, 2, 3,..., n are composed so that an inquiry signal and a reply signal may be time inverse with each other and the sensor elements 1, 2, 3,..., n may have different SAW propagation time.