A method of controlling a spectroscopic module that includes a measurement light source, a variable-wavelength optical filter, a photodiode, and a conversion circuit for converting a drive signal voltage into a gap displacement amount. The spectroscopic module has a reference light source for emitting a reference light beam of a known wavelength. The controlling method involves varying a gap for the incident reference light beam, extracting two maximum points among data output from the photodiode, and updating a first conversion formula provided in the conversion circuit through use of drive signal voltages and gap amounts corresponding to the two points.