An atomic absorption spectrophotometer including a self reverse type hollow cathode lamp and a photomultiplier tube further includes a preliminary tester for measuring a strength of a signal L when a smaller current is supplied to the hollow cathode lamp and a strength of a signal H when a larger current is supplied while a voltage V applied between a cathode and an anode of the photomultiplier is changed. It also includes an optimal voltage detector for detecting a value V0 of the voltage V at which a super ratio U (which is defined as a ratio of a first ratio L0/H0 to a second ratio L1/H1) is closest to unity under a condition that two values V1 and V0 of the voltage V are arbitrarily chosen so that a ratio H1/H0 or a ratio L1/L0 is a predetermined value. The value V0 is used in the following proper measurement of a sample analysis to apply voltage V to the photomultiplier tube of the photometric detector, where a larger current and a smaller current are alternately supplied to the hollow cathode lamp to perform the background correction. This automatically improves the accuracy of the background correction.