PURPOSE: To obtain a high frequency power supply device whose output power is varied with good control performance to an output power setting value with simple constitution by providing a gain control section whose gain is changed so that the relation between an output power setting value of an output power setting section and an output power from a power amplifier is linear.

CONSTITUTION: A variable voltage gain amplifier section 3 consists of an operational amplifier 31, a fixed resistor 32 whose resistance is R0, a photoresistor element 33 comprising a light emitting diode 33a and a variable resistor 33b whose resistance R1 is changed, a fixed resistor 34 whose resistance if R2 and an operational amplifier 36. Denoting the setting voltage of an output power setting section 1 as E (DC value). Then an outputted current (I) from the operational amplifier 1 is expressed as equation I. Since the resistor 33b of the photoresistor element 33 changes as expressed in equation (II), the result is expressed in equation (III). In supplying a high frequency signal V1 from a high frequency oscillator 2, the output voltage V2 is expressed as equation (V) through the relation in equation (IV). Thus, in giving the output voltage V2 to the power amplifier 4, the high frequency power P outputted from the power amplifier 4 is expressed in equation (VI) and is proportional to the setting voltage E.