Abstract: Leak-detecting apparatus for a tire-vulcanizing press which is extremely sensitive and is capable of monitoring minute leaks in a diaphragm or bag in the region of the bottom of the bag susceptible to cracking and removed from the vicinity of the leak-detecting apparatus which ordinarily is at a top plate of the press. The apparatus includes temperature-sensitive resistive means in a normally balanced bridge circuit which is unbalanced by leakage fluid from the diaphragm for indicating a malfunction of the press and also includes a check valve for preventing air from outside the press from entering the vicinity of the temperature-sensitive resistive means.