a mass spectrometer comprising an ionization means for ionizing sample compounds to be analyzed mass spectro - scopically in an atmospheric pressure , a sample solution supply means for supplying a solution containing the sample compounds to the ionization means , means for feeding the ions formed by the ionization means through an aperture disposed in an electrode into a vacuum region and a ion trap type mass spectroscopic means for mass spectroscopically analyzing ions entered through the aperture into the vacuum region , in which an ion decelerating electric field forming means is disposed between the electrode disposed with the aperture and an electrode disposed with an ion entrance opening for entering the ions into the ion trap type mass spectroscopic means for forming an electric field for decelerating the ions , and the ions injected to the ion trap mass spectroscopic means is lowered . this facilitates accumulation ions in the ion trap mass spectralyzing means even if a high drift voltage is used thereby enabling high sensitivity analysis for polar compounds such as peptides .