To highly precisely measure the three-dimensional positions of a swimming body swimming in a water vessel, using an ultrasonic wave.

A radio wave transmitter 18 transmits a transmission command to an artificial fish 3 inside the water vessel 1. A radio wave receiver 5 inside the fish 3 outputs the received transmission command to an ultrasonic transmitter 8 to transmit an ultrasonic transmission pulse T. The pulse T is received by receivers 11a-11c inside the water vessel 1, and received signals A1-A3 are input to a propagation time clock generator 14. The generator 14 generates propagation time clock pulses C1-C3 corresponding to ultrasonic propagation times t1-t3 from the fish 3 upto the receivers 11a-11c. The pulses C1-C3 are counted by a pulse counter 16, and counted values are input to an operation control part 17, and the propagation times t1-t3 are found to calculate three- dimensional positions x, y, z of the fish 3.