A nebulized jet from a nebulizer is subjected to mechanical agitation and stirring to render the mists uniform and fine, then the droplets of the mists thus rendered uniform and fine are ionized. By accelerative collision of the resulting ions in a medium pressure chamber it is made possible to effect desolvation efficiently. Consequently, chemical noises caused by cluster ions can be greatly diminished and it is possible to attain high sensitive measurement of a sample in atmospheric pressure ionizations LC/MS.