An apparatus for quantitative secondary ion mass spectrometry comprising a sealed chamber for storing a sample containing a light impurity element which is to be analyzed, secondary ion generating means for bombarding a primary ion beam onto the sample so as to allow the sample to emit a secondary ion of the light element, and quantitative analyzing means for detecting the secondary ion so as to quantitatively analyze the light element contained in the sample. First evacuating means evacuates said sealed chamber to an ultrahigh vacuum during quantitative analysis. First cryopanel means is arranged to surround the sample, and first cooling means keeps said first cryopanel means at a cryogenic temperature during quantitative analysis so that said first cryopanel means adsorbs a gas present in said sealed chamber.