Abstract: Provided is a negative ion source and a negative ion generation method capable of providing a high negative ion generation efficiency. A negative ion source includes a housing that includes: an inlet from which a sample is introduced; a plasma generation region communicated with the inlet, a plasma being generated by discharge in the plasma generation region; a negative ion generation region in which particles dissociated or excited by a reaction of the generated plasma with the sample are converted into negative ions; and an extraction port communicated with the negative ion generation region, the generated negative ions being extracted outside through the extraction port. The negative ion generation region is filled with a thermionic emission material for generating thermoelectrons by high frequency heating.