A derivative spectroscopy system for achieving a tunable resolution of 2 nm or less in resolving spectral components of an input optical signal is provided so as to estimate derivative spectra of the input optical signal based on the resolved spectral components. In the system, a first dispersive-element structure spectrally decomposes the input optical signal into subband signals. A second dispersive-element structure receives part or all of the subband signals and spectrally decomposes the received subband signals to plural spectral components. A material having a temperature-variant refractive index is used to build the second dispersive-element structure, enabling a shift of center wavelength of each spectral component as small as 2 nm of less upon changing a temperature of the second dispersive-element structure. By obtaining three spectral-component sets obtained at three different predetermined temperatures with the center-wavelength shift of 2 nm or less, first- and second-order derivative spectra are obtained with good accuracy.