A method of controlling a spectroscopic module for dispersing a measurement light beam and detecting dispersed light, the spectroscopic module comprising: a measurement light source configured to emit the measurement light beam; a reference light source configured to emit a reference light beam of a known single wavelength; a variable-wavelength optical filter configured to output an interference light beam based on the measurement light beam or reference light beam by multiple reflection; and a photodiode configured to detect the interference light beam, wherein the variable-wavelength optical filter includes a pair of opposed reflectors (201, 202) and a piezoelectric driver that lets a gap between the pair of reflectors (201, 202) undergo displacement by receiving of a drive signal, and the piezoelectric driver controls the gap and the spectroscopic module has a conversion circuit configured to convert a voltage of the drive signal into gap displacement data by a first conversion formula and to convert the gap displacement data into data about an interference wavelength of the variable-wavelength optical filter by a second conversion formula, characterized in that the method comprising: detecting three maximum points corresponding to orders of the interference light beam based on the reference light beam from the photodiode; updating the first conversion formula through use of results obtained from two points; determining an absolute value of the gap without knowledge of the interference orders by detecting the three maximum points; and alternating dispersing the measurement light beam and rewriting the first conversion formula by the reference light beam. a measurement light source configured to emit the measurement light beam; a reference light source configured to emit a reference light beam of a known single wavelength; a variable-wavelength optical filter configured to output an interference light beam based on the measurement light beam or reference light beam by multiple reflection; and a photodiode configured to detect the interference light beam, wherein the variable-wavelength optical filter includes a pair of opposed reflectors (201, 202) and a piezoelectric driver that lets a gap between the pair of reflectors (201, 202) undergo displacement by receiving of a drive signal, and the piezoelectric driver controls the gap and the spectroscopic module has a conversion circuit configured to convert a voltage of the drive signal into gap displacement data by a first conversion formula and to convert the gap displacement data into data about an interference wavelength of the variable-wavelength optical filter by a second conversion formula, characterized in that the method comprising: detecting three maximum points corresponding to orders of the interference light beam based on the reference light beam from the photodiode; updating the first conversion formula through use of results obtained from two points; determining an absolute value of the gap without knowledge of the interference orders by detecting the three maximum points; and alternating dispersing the measurement light beam and rewriting the first conversion formula by the reference light beam. detecting three maximum points corresponding to orders of the interference light beam based on the reference light beam from the photodiode; updating the first conversion formula through use of results obtained from two points; determining an absolute value of the gap without knowledge of the interference orders by detecting the three maximum points; and alternating dispersing the measurement light beam and rewriting the first conversion formula by the reference light beam.