A measurement controller (20): computes the position of a plane (S1) representing a side surface of a work implement (1A) in an image-capturing-device coordinate system (Co1) on the basis of an image of the side surface of the work implement captured by an image-capturing device (19) and an internal parameter of the image-capturing device; computes the coordinate values of a point on the work implement in the image-capturing-device coordinate system (Co1), the point corresponding to any pixel constituting the work implement on the captured image, on the basis of positional information on the pixel on the captured image and the position of the plane (S1); and converts the coordinate values of the point on the work implement in the image-capturing-device coordinate system, the point corresponding to the pixel, to coordinate values in a work-implement coordinate system (Co3) to output the coordinate values in the work-implement coordinate system (Co3) to a work-machine controller (50) of a hydraulic excavator (1).