A tunable filter (100, 102) comprising: a polarization splitter that splits input light into two linearly polarized light rays of mutually orthogonal vibration directions; a wavelength dispersion spectroscopic element that splits the two linearly polarized light rays split by the polarization splitter, into two spectral images having spatial spread in one direction, the two spectral images corresponding to the two linearly polarized light rays; and a reflective spatial modulator device that modulates and reflects linearly polarized light in each wavelength region for the two spectral images independently from each other, wherein the modulated light reflected at the reflective spatial modulator device reenters the wavelength dispersion spectroscopic element and the polarization splitter, thereby splitting and outputting the modulated light, as output light in a wavelength region modulated by the reflective spatial modulator device and output light in a wavelength region not modulated, input light and reentered light to the polarization splitter and input light and reentered light to the wavelength dispersion spectroscopic element are parallel light fluxes, and characterized in that the tunable filter (100, 102) further comprises: a pair of optical systems (130, 144) provided between the polarization splitter and the wavelength dispersion spectroscopic element, so that a rear-side focal position of one optical system of the pair matches a front-side focal position of the other optical system of the pair; an optical system provided between the wavelength dispersion spectroscopic element and the reflective spatial modulator device, the optical system provided with the wavelength dispersion spectroscopic element at a front-side focal plane and the reflective spatial modulator device at a rear-side focal plane; and an input/output slit provided at a rear-side focal plane of the one optical system of the pair, the two linearly polarized light rays being inputted to and outputted from the input/output slit, wherein a polarization split point of the polarization splitter matches a front-side focal plane of the one optical system of the pair. a polarization splitter that splits input light into two linearly polarized light rays of mutually orthogonal vibration directions; a wavelength dispersion spectroscopic element that splits the two linearly polarized light rays split by the polarization splitter, into two spectral images having spatial spread in one direction, the two spectral images corresponding to the two linearly polarized light rays; and a reflective spatial modulator device that modulates and reflects linearly polarized light in each wavelength region for the two spectral images independently from each other, wherein the modulated light reflected at the reflective spatial modulator device reenters the wavelength dispersion spectroscopic element and the polarization splitter, thereby splitting and outputting the modulated light, as output light in a wavelength region modulated by the reflective spatial modulator device and output light in a wavelength region not modulated, input light and reentered light to the polarization splitter and input light and reentered light to the wavelength dispersion spectroscopic element are parallel light fluxes, and characterized in that the tunable filter (100, 102) further comprises: a pair of optical systems (130, 144) provided between the polarization splitter and the wavelength dispersion spectroscopic element, so that a rear-side focal position of one optical system of the pair matches a front-side focal position of the other optical system of the pair; an optical system provided between the wavelength dispersion spectroscopic element and the reflective spatial modulator device, the optical system provided with the wavelength dispersion spectroscopic element at a front-side focal plane and the reflective spatial modulator device at a rear-side focal plane; and an input/output slit provided at a rear-side focal plane of the one optical system of the pair, the two linearly polarized light rays being inputted to and outputted from the input/output slit, wherein a polarization split point of the polarization splitter matches a front-side focal plane of the one optical system of the pair. a pair of optical systems (130, 144) provided between the polarization splitter and the wavelength dispersion spectroscopic element, so that a rear-side focal position of one optical system of the pair matches a front-side focal position of the other optical system of the pair; an optical system provided between the wavelength dispersion spectroscopic element and the reflective spatial modulator device, the optical system provided with the wavelength dispersion spectroscopic element at a front-side focal plane and the reflective spatial modulator device at a rear-side focal plane; and an input/output slit provided at a rear-side focal plane of the one optical system of the pair, the two linearly polarized light rays being inputted to and outputted from the input/output slit, wherein a polarization split point of the polarization splitter matches a front-side focal plane of the one optical system of the pair.