In the ionization probe, an assist gas nozzle, coaxial with a nozzle, is provided around a nozzle that discharges a liquid sample. A housing inside which is formed an annular assist gas channel is provided around the assist gas nozzle. A gas inlet for introducing gas into the assist gas heating channel and gas outlet for feeding gas from channel to assist gas nozzle are provided at opposite locations across the center of the channel. Gas introduced into the assist gas heating channel is heated by a substantially annular heater, and high temperature gas is discharged through gas discharge hole of the assist gas nozzle. The heater and the channel through which high temperature gas flows are arranged solely at the tip part of the ionization probe, so thermal insulation of the probe base part can be easily accomplished.