A mass spectrometer has an ionizing portion for ionizing a sample, a mass spectrometric portion for subjecting a sample ion to mass separation and detecting the sample ion and a control portion for controlling operation of a total of the apparatus. The mass spectrometer is provided with a colliding portion provided at an ion path until a sample ion generated by the ionizing portion is introduced to amass spectrometric portion and a gas introducing portion for introducing one kind or more of gases selected from two kinds or more of gases into the colliding portion. By carrying out analysis by using the apparatus, highly accurate mass spectrometry can be carried out.