PURPOSE: To detect lack of cooling water in water passage and to prevent mass-production of deteriorated products by arranging a water passage communicating with a cooling water passage for a die in inner part of a main body and connecting a cooling water detecting means for the water passage with electric circuit for transmitting output signal to an alarm.

CONSTITUTION: In the main bodies 1 of the detectors A, A', the water passages 4 corresponding to the cooling water passages formed in the inner parts of a fixed die 2 and a movable die 3 are penetrated. In upper and lower ends of the water passage 4, hoses 6, 8 are connected with a flow-in metal fitting 5 and flow-out metal fitting 7. On the way of the water passage 4, the detecting means 10 for detecting whether the cooling water flows or not, is arranged and connected with the electric circuit 16. Then, in the case the cooling water flows in the water passage 4, the detecting rods 10a, 10b are electrically connected and the electric circuit 16 turns off and the alarm 17 does not work. In the case the cooling water does not flow, the detecting rods 10a, 10b becomes insulating state and the electric circuit 16 is turned on and the alarm 17 works. Therefore, there is no chance, which lack of the cooling water flow is not noticed.