To provide a mechanism in which ions in the vicinity of a surface of a sample board are collected to the maximum in a desorption ionization method.

An ion capturing port having a width and a height equal to or more than spread of generated ions is installed and is positioned so as to be contacted with the sample board. Alternatively, a face lower than that on which a sample is placed is installed by providing level difference on the sample board, and an opening bottom face of the ion capturing port having the width and the height equal to or more than the spread of the generated ions is installed lower than the face on which the sample is loaded.