In a mass analyzer, an acceleration voltage is applied to an ionization chamber in which a voltage to be applied to repeller electrodes to repel ions is generated with reference to the acceleration voltage and a repeller voltage source is linked with a controller by use of a photocoupler or the like such that the repeller voltage is adjusted with the repeller voltage source electrically insulated from the controller. The repeller voltage can be easily and automatically adjusted by use of a low-voltage control signal so as to obtain a sensitivity developing the maximum value of the quantity of detected ions and hence a qualitative or quantitative analysis of a sample can be achieved with a high sensitivity. The controller processes digital signals and appropriate parameters are stored in a memory. Each parameter is changed with the center of change set to an appropriate parameter previously used and consequently the automatic control is simplified.