PURPOSE: To protect a compressor with fine lubricating performance being maintained by converting the frequency of an ordinary commercial power source into necessitated frequency after each signal for bearing temp., rotating speed, etc. in addition to a detected room temp. is included.

CONSTITUTION: Room temp. (t), compressor bearing temp. TS, compressor lubricating temp. TO, and a compressor rotating speed N are detected by a temp. sensor 9, a thermister 10, a thermister 11, a rotating speed sensor 12 respectively. When there is a difference between detected temp. t and the setting temp. tO, supplied frequency is selected to bring t near to tO. When t is nearly equal to tO, the previous frequency is kept. After TS/TO is compared with a setting value K, supplied frequency is the same as before in case of TS/TO<K. In case of TS/TO≥K, the case is considered to be just before seizure and after N is compared with the setting rotating speed NO, supplied frequency is determined to frequency corresponding to a rotating speed N+n (n: finely cut constant) in case of N<NO. In case of N≥NO, supplied frequency is fixed to frequency corresponding to a rotating speed N-n. A frequency conversion part receives said changes to convert the frequency of an ordinary power source into necessitated frequency and applies it to a compressor 2.