An ionizer 1 including an ionization chamber 10, a sample gas introduction port 14 provided in the ionization chamber 10 for introducing sample gas, an electron beam emitting section 11 which emits an electron beam toward the ionization chamber 10, electron beam passage openings 10a and 10b which are formed on a path of the electron beam emitted from the electron beam emitting section 11 on a wall of the ionization chamber 10 and has a length in a direction of the path longer than a width of a cross section orthogonal to the direction, and an ion outlet 10c provided in the ionization chamber 10 for emitting an ion of the sample gas generated by irradiation with the electron beam, and a mass spectrometer 60 including the ionizer 1.