A spray area in which a large number of droplets of a liquid sample sprayed from a spray nozzle is separated from the tip of a needle electrode for corona discharge by a sufficiently large distance, with a grid electrode facing the needle electrode placed in between. Ring electrodes for creating an electric field which drives primary ions that should react with the sample and generate sample-derived ions are provided within an ion chamber between the grid electrode and the spray area. Primary ions generated by corona discharge within the space between the needle electrode and the grid electrode pass through the opening of the grid electrode, reach the spray area under the effect of the electric field, and ionize sample components. Since the droplets are prevented from adhering to the needle electrode, the corona discharge is maintained in a stable state.
