A photodetector output correction method used for a spectroscopic analyzer or a spectroscope comprising: a light source, an interferometer splitting and then synthesizing light beams emitted from the light source, and a photodetector for measuring the intensity of light beams exiting from the interferometer, wherein the photodetector is adapted such that a relationship between incident light intensity and an output value is substantially linear within a predetermined intensity range and is nonlinear out of the predetermined intensity range, the photodetector output correction method comprising: making the light beams emitted from the light source enter the photodetector interposing an optical element having a known wavenumber transmission characteristic or a known wavenumber reflection characteristic so as to make intensity of the light beams fall within the predetermined intensity range, and acquiring a first output value as an output value of the photodetector with respect to each of light beams that have predetermined multiple wavenumbers and are included in the incident light beams; making the light beams emitted from the light source enter the photodetector without interposing the optical element, and acquiring a second output value as an output value of the photodetector with respect to each of light beams that respectively have the predetermined wavenumbers and are included in the incident light beams; and using a ratio between the first output value and the second output value at each of the predetermined wavenumbers and the wavenumber transmission characteristic or wavenumber reflection characteristic of the optical element as parameters to obtain an arithmetic expression for calculating intensity of incident light beams from an output value of the photodetector, wherein given that the arithmetic expression is C(y), the arithmetic expression C(y) is determined such that a value of the following Expression 1 falls within a predetermined range around F(k) that is a value indicating the wavenumber transmission characteristic or the wavenumber reflection characteristic of the optical element, Y 1 k / C y ⋅ Y 2 k where k represents a wavenumber, Y 1 (k) represents the first output value, Y 2 (k) represents the second output value, F(k) represents the wavenumber transmission characteristic or the wavenumber reflection characteristic of the optical element, y represents the output value of the photodetector, and C(y) represents the arithmetic expression. making the light beams emitted from the light source enter the photodetector interposing an optical element having a known wavenumber transmission characteristic or a known wavenumber reflection characteristic so as to make intensity of the light beams fall within the predetermined intensity range, and acquiring a first output value as an output value of the photodetector with respect to each of light beams that have predetermined multiple wavenumbers and are included in the incident light beams; making the light beams emitted from the light source enter the photodetector without interposing the optical element, and acquiring a second output value as an output value of the photodetector with respect to each of light beams that respectively have the predetermined wavenumbers and are included in the incident light beams; and using a ratio between the first output value and the second output value at each of the predetermined wavenumbers and the wavenumber transmission characteristic or wavenumber reflection characteristic of the optical element as parameters to obtain an arithmetic expression for calculating intensity of incident light beams from an output value of the photodetector, wherein given that the arithmetic expression is C(y), the arithmetic expression C(y) is determined such that a value of the following Expression 1 falls within a predetermined range around F(k) that is a value indicating the wavenumber transmission characteristic or the wavenumber reflection characteristic of the optical element, Y 1 k / C y ⋅ Y 2 k where k represents a wavenumber, Y 1 (k) represents the first output value, Y 2 (k) represents the second output value, F(k) represents the wavenumber transmission characteristic or the wavenumber reflection characteristic of the optical element, y represents the output value of the photodetector, and C(y) represents the arithmetic expression.