A method of correcting a measurement value of a laser scanner, comprising: in a laser scanner including a light emitting unit configured to emit a distance measuring light, a light receiving unit configured to receive a reflected distance measuring light, a distance measuring unit configured to perform a distance measurement based on an output of the light receiving unit, an optical axis deflecting unit having at least a pair of prisms (RB, RA) configured to deflect and emit the distance measuring light from an emission optical axis, and deflect the reflected distance measuring light onto a light receiving optical axis, and an emitting direction detecting unit configured to detect a deflection angle and an emitting direction of the distance measuring light from a rotation angle of the optical axis deflecting unit, (a) a step of measuring a distance to a measurement point by the distance measuring unit; (b) a step of detecting rotation angles of the prisms by the emitting direction detecting unit; and (c) a step of obtaining, based on rotation angles of the prisms, a true distance measurement value corrected for a length of an optical path length difference in light emission and/or light reception occurring according to the rotation angles of the prisms (RA,RB) by subtracting the optical path length difference from a distance measurement value of the distance measuring unit in a laser scanner including a light emitting unit configured to emit a distance measuring light, a light receiving unit configured to receive a reflected distance measuring light, a distance measuring unit configured to perform a distance measurement based on an output of the light receiving unit, an optical axis deflecting unit having at least a pair of prisms (RB, RA) configured to deflect and emit the distance measuring light from an emission optical axis, and deflect the reflected distance measuring light onto a light receiving optical axis, and an emitting direction detecting unit configured to detect a deflection angle and an emitting direction of the distance measuring light from a rotation angle of the optical axis deflecting unit, (a) a step of measuring a distance to a measurement point by the distance measuring unit; (b) a step of detecting rotation angles of the prisms by the emitting direction detecting unit; and (c) a step of obtaining, based on rotation angles of the prisms, a true distance measurement value corrected for a length of an optical path length difference in light emission and/or light reception occurring according to the rotation angles of the prisms (RA,RB) by subtracting the optical path length difference from a distance measurement value of the distance measuring unit (a) a step of measuring a distance to a measurement point by the distance measuring unit; (b) a step of detecting rotation angles of the prisms by the emitting direction detecting unit; and (c) a step of obtaining, based on rotation angles of the prisms, a true distance measurement value corrected for a length of an optical path length difference in light emission and/or light reception occurring according to the rotation angles of the prisms (RA,RB) by subtracting the optical path length difference from a distance measurement value of the distance measuring unit