PURPOSE: To detect that a cooling pipe is clogged or not in its early stage by a method wherein a water gathering chamber is provided on the upper part of a water intake pipe, and the level of water surface in the water gather ing chamber is detected.

CONSTITUTION: When cooling water 13 is spouted out downwards from a pressure control pipe 9, it gathers partially in a sump chamber 11. By this process, a float 12 floats turning around a support pin 18, so that a switch 14 is turned ON. Therefore, leads 15 are short-circuited, and an alarm 21 sounds a buzzer or outputs anomalous signals receiving a contact signal to inform that the cooling pipe is clogged. Even if water gathers in the sump chamber 11, it is discharged out of the chamber 11 a little later through a drain hole 16 provided to the sump chamber 11, so that it is found that cooling water is continuously spouted out from the pressure control tube or not. Cooling water 13 gathered in the sump chamber 11 is not required to be discharged out each time. By this setup, it is judged that cooling water is continuously or instantaneously spouted out from the pressure control pipe, so that it is found that a cooling tube is clogged or not.