PURPOSE: To effectively ionize a sample liquid sent to a porous member, so as to improve sensitivity by making the transmission capability of the sample liquid in the center of the porous member larger than around the porous member.

CONSTITUTION: The transmission capability of a sample liquid in the center of a porous member 11 mounted on an ionization chamber, is defined larger than around the porous member. The most part of the sample liquid passes the center part of the porous member 11, and is thereby infused to the surface of the center part, and is ionized by bombardment of a primary particle beam. Thus the most part of the sample liquid sent to the porous member 11 is ionized, and an ion source in which a sample liquid does not stay in the porous member 11, can be offered.