To provide a power supply voltage compensator which enables compensation of the deterioration of the performance of a device to be compensated (for instance a magnetic disc device), even if power supply voltage fluctuates.

A power supply voltage which is supplied at a certain specified voltage value (for instance 5 V±10%) is detected by a power supply voltage fluctuations detecting means (dynamic power supply voltage range detecting means) 1. Meanwhile, optimum parameters corresponding to respective power supply voltages are stores in a memory means 3 before hand. The parameter corresponding to the voltage detected by a power supply voltage fluctuations detecting means is selected to make a compensation object device (signal processing means) 4 operate.