A displacement detecting device for detecting relative displacement in one direction of a measuring object by a movement of a diffraction grating, comprising: a light source for emitting coherent light; a collimate lens for making the coherent light emitted from the light source into parallel light flux; a light flux splitting element for splitting collimate light, which has been made into parallel light flux by the collimate lens, into two light fluxes; deflection mirrors (8, 9) for deflecting each of split light fluxes to enter the diffraction grating at prescribed angle; two lenses (14, 20) for correcting light path with equal focus distance arranged in light path of the light fluxes; a prism unit for shifting light path to reenter first diffracted light, which is diffracted in the diffraction grating, into the diffraction grating by moving in parallel the light path of the first diffracted light to a vertical direction with respect to a measuring direction; an interfered light receiving unit for receiving light by interfering diffracted light, which was moved in parallel in the prism unit for shifting light path, re-diffracted in the diffraction grating and superposed in the light flux splitting element; and a relative position detector for outputting displacement information of the diffraction grating based on interfered light intensity, wherein each of the light fluxes split in two by the light flux splitting element enters the diffraction grating at incident angle that diffraction angle will be approximately vertical to the diffraction surface of the diffraction grating and such that the light fluxes will be symmetrical with normal line of grating surface of the diffraction grating as symmetrical axis, and the two lenses (14, 20) for correcting light path are arranged on the symmetrical axis, and one focus of one lens for correcting light path is on the grating surface, and another focus of one lens for correcting light path is arranged to coincide with focus of another lens for correcting light path. a light source for emitting coherent light; a collimate lens for making the coherent light emitted from the light source into parallel light flux; a light flux splitting element for splitting collimate light, which has been made into parallel light flux by the collimate lens, into two light fluxes; deflection mirrors (8, 9) for deflecting each of split light fluxes to enter the diffraction grating at prescribed angle; two lenses (14, 20) for correcting light path with equal focus distance arranged in light path of the light fluxes; a prism unit for shifting light path to reenter first diffracted light, which is diffracted in the diffraction grating, into the diffraction grating by moving in parallel the light path of the first diffracted light to a vertical direction with respect to a measuring direction; an interfered light receiving unit for receiving light by interfering diffracted light, which was moved in parallel in the prism unit for shifting light path, re-diffracted in the diffraction grating and superposed in the light flux splitting element; and a relative position detector for outputting displacement information of the diffraction grating based on interfered light intensity, wherein each of the light fluxes split in two by the light flux splitting element enters the diffraction grating at incident angle that diffraction angle will be approximately vertical to the diffraction surface of the diffraction grating and such that the light fluxes will be symmetrical with normal line of grating surface of the diffraction grating as symmetrical axis, and the two lenses (14, 20) for correcting light path are arranged on the symmetrical axis, and one focus of one lens for correcting light path is on the grating surface, and another focus of one lens for correcting light path is arranged to coincide with focus of another lens for correcting light path.