PURPOSE: To prevent the damage of machinery due to overload by detecting the molding power of many kinds of molding tools and utilizing the results for automatic control of molding.

CONSTITUTION: Opposite lateral surfaces positioned in diametral symmetry are cut away by parallel planes 12' and 12" and cross gages are provided on cross lines 14', 14", 16' and 16" of planes 14 and 16 suffering shear with the cut planes, whereby the shear strain is detected, while output and input lead wires are passed through gaps between the parallel planes 12' and 12" and the circumference of a pin. The device is constituted so that an alarm is given before a load whereby the shear pin shears is applied or that the input to a machine is cut, whereby the pin can be prevented from being damaged and be employed again.