An ionization detector which ionizes particles of a substance to be easily ionized and then measures the number of the ionized particles as ionization current. The detector comprises a heater for ionizing particles, a collector disposed opposite to the heater for collecting ionized particles, and a control arrangement for controlling the temperature of the heater to a constant value. Thus, by preventing the fluctuation of the temperature of the heater due to the fluctuations of the voltage of the power source and of the flow of a cooling fluid which is detected as the ionized particles, an exact measurement of the number or concentration of ionized particles can be performed.