First and second output signals are generated based on amounts of light received by first and second light-receiving elements, respectively, when a detection head is moved along a scale; a first amplitude adjustment is performed to adjust an amplitude of the first or second output signal such that levels of first and second output signals become equal at a first reference phase; a second amplitude adjustment is performed to adjust an amplitude of the first or second output signal such that levels of the first and second output signals become equal at a second reference phase; and a reference signal that starts at a timing when levels of the second output signal and the first output signal subjected to the first amplitude adjustment become equal and ends at a timing when levels of the second output signal and the first output signal subjected to the second amplitude adjustment become equal.