To hold a clearance between a point which current is zero-crossed and the peak point of a positioning signal to a constant value by respectively detecting the moving position of a piston in a compressor driven through an inverter and current which flows in the compressor, and outputting a pulse width modulation signal on the basis of both detecting signals.

AC voltage AC is outputted to an inverter 20 after the voltage AC is rectified and smoothed to dc voltage DC by a rectifying and smoothing means 10, the dc voltage DC is converted to the ac voltage AC, and the dc voltage DC is applied on a linear compressor 30 so as to carry out compression. In this case, the position of a piston in a compressor 30 is sensitized by a position sensitizing means 40, and also current which transfers to the compressor 30 is sensitized from a current transformer by a current sensitizing means 50, and its sensitizing signal is outputted to a control unit 60. A pulse width modulation signal Ps is generated on the basis of a position signal Sp and current I, the signal Ps is outputted to the inverter 20, the dc voltage DC is converted to the ac voltage AC having a desired frequency, and it is applied to the compressor 30.