PURPOSE: To easily handle a device and avoid large size and complication of the device, by constituting the device such that a cesium compound stable in a normal environment is used and mixed with a reducing agent then charged in an ion source while heated to generate metal cesium.

CONSTITUTION: A switch is turned on to feed a heating current to a heater coil 3 from a power source 5 and heat a vessel to about 700°C. In this way, cesium chloride CsC1 causes reducing action with calcium Ca to generate cesium gas Cs. This cesium gas reaches a low temperature part, that is, a hole 10 in a ceramic base body 1, upper part of a reservoir 8 and filter 9, and is cooled, liquefied, extended through a filter and dropped in the reservoir 8. In this way, metal cesium is pooled in the reservoir. Then a switch 11 is turned on to heat the metal cesium pooled in the reservoir to fixed temperature from several tens to several hundreds of degrees. Under this condition, negative high voltage is applied to electrodes 13, 14 by power sources 15, 16 for an emitter electrode 10, then the cesium is evaporated and ionized.