The present report deals with the reading-out apparatus of the towing carriage speed of the experimental tank, using Dekatron that is a kind of the cold counting tube of glow discharge type. The apparatus consists of five parts ; the pick up of electromagnetic type, the counting adjustor, the timer, the counting indicator and the power source. The counting adjustor is composed of a gate circuit, an amplifier, a reset circuit and a reserving circuit of the counting indicator and the timer is a combimation of a standard oscillator of 1, 000 cycles and a counting unit. The measurement of the speed during one second and the indication of it during 0.8 second are repeated alternately. If we desire to keep the indication unchangeable, it can be done by the reserving circuit. Thses operations being worked out electrically, there is no room for entering of individual error, time lag and etc., As to error there may be considered three causes, the unavoidable miscount of ± 1, drift of standard frequency and a characteristic in construction of the pick up. At usual speed, however, the error remains within 0. 300. 15 %. Besides, the counting indicator for measuring propeller revolution is attached to the apparatus.