PURPOSE: To make blocking oscillation generating when temperature exceeds a prescribed value by inserting a heat-sensitive thyristor into the gate circuit of PUT of a relaxation oscillation circuit.

CONSTITUTION: The heat-sensitive thyristor 3 is inserted into the gate circuit of PUT2 of the blocking oscillation circuit. In the case when a circuit as shown by the figure is used, for instance, the heat-sensitive thyristor 3 is in the state OFF when the temperature to be detected is low, and the potential of the gate is nearly equal to the voltage of a DC power source 1. Therefore, even when a capacitor 4 is charged, PUT2 does not turn into the state ON since the gate is reversely biased, thereby not making blocking oscillation. When the temperature rises up to a prescribed value, the heat-sensitive thyristor 3 turns into the state ON, the gate of PUT2 is biased by the voltage set by resistors 7 and 8, and the circuit performs blocking oscillation. The heat-sensitive thyristor 3 which is turned into the state ON by the rise of temperature does not return to the state OFF even when the temperature lowers below the prescribed value and thus the blocking oscillation continues. Accordingly, it continues to generate pulse signals at both ends of a load 6 after the arrival of the temperature at the prescribed value irrespective of the rise and fall of temperature.