The present invention has been accomplished to provide an atomic absorption spectrophotometer capable of obtaining measurement data always in the state where the lowest detection limit performance is optimized, without depending on the frequency of the power supply. In a control program which runs on the microcomputer chip 42 mounted on the atomic absorption spectrophotometer 110, a plurality of lighting periods of the light sources 11 and 12 and extraction periods of the sampling data are memorized, whose lowest detection limit performance are optimized for the frequencies (50 Hz and 60 Hz) of the AC power source for driving the AC motor 22. In using the apparatus, by the control program, the frequency of the power source used in this apparatus is identified, the lighting period and sampling data extraction period corresponding to the identified frequency and the measurement mode that a user of the apparatus has previously set are selected from among a plurality of memorized values, and the appropriate lighting period is set to the hardware (PLD 43). Accordingly, without depending on the frequency, it is possible to obtain measurement data always in the state where the lowest detection limit performance is optimized.