In a quadrupole mass spectrometer which includes four parallel rod electrodes between opposite pairs of which overlapping voltages.+-.(U+V cos .omega. t) are applied (where U: a continuous voltage and V cos .omega. t: a radio-frequency voltage) and which effects mass-analysis by an electric field formed within the four parallel rod electrodes, at least one parameter of U, V and .omega. has two different values, and the values are changed over to select the first stable region or the second stable region, the first stable region being nearest to the original point and the second stable region being next near to the original point on the a - q plane (where a:ordinate, q:abscissa) showing "stability diagram", and the ions being able to pass through the four parallel electrodes under the conditions of the first and second stable regions, where the variable a=8eU/mr.sub.O.sup.2 .omega..sup.2, q=4eV/mr.sub.O.sup.2 .omega..sup.2, U: level of DC voltage, V: peak of the radio-frequency voltage, e: charge of ion, m:mass of ion, r.sub.O : radius of an inscribed circle of the four parallel rod electrodes and.omega.: angular frequency of the radio frequency voltage.