PURPOSE: To contrive early discovery of trouble in a compressor by converting a temperature in a predetermined location of the compressor into voltage by a temperature sensor and judging the compressor abnormal to generate an alarm when this voltage exceeds the reference set voltage.

CONSTITUTION: A temperature sensor 10 is mounted to a predetermined location of a compressor 1 in which a piston 6 is reciprocated in a cylinder case 3 through a connecting rod 7 by rotating a crankshaft 5. Here an air temperature in a crank case 2 is measured by protruding the temperature sensor 10, for instance, into the crank case 2. An output signal of this temperature sensor 10 is input to an abnormality detector 11, and voltage (compared voltage) in proportion to the temperature is compared with the reference set voltage in a comparator circuit. Here output of the comparator circuits assumed H, and an alarm output is fed through an FF circuit by this output H when the compared voltage exceeds the reference set voltage.