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 and a piezoelectric driver that lets a gap between the pair of reflectors 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 and a piezoelectric driver that lets a gap between the pair of reflectors 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 . <EOS>