PURPOSE: To determine the direction of water flow by inserting an apparatus into a pipe at a branch of an underground fire hydrant and determining a phototransistor for which the light is intercepted.

CONSTITUTION: A slit 2 is made at the front end of a body 1, being inserted into a channel in the direction intersection the flow, while traversing the body 1 in the direction of the channel. A pair of LEDs 4, 4 are mounted on one inner wall of the slit 2 while spaced apart in the direction of the channel and phototransistors 6, 6 are disposed oppositely to the LEDs 4, 4 on the opposite sides of the slit 2. A shade 14 moving along the slit 2 by the function of the flow to intercept the signal from the LED 4 is disposed in the slit 2.