A height measuring device includes a light source that emits light in a direction oblique to a top surface of a specimen, a slit that shapes the light from the light source to form a slit image on the specimen, an imaging element that detects reflected light reflected by the specimen, and an arithmetic unit. The arithmetic unit: identifies a slit image of the reflected light reflected by the top surface of the specimen from among a plurality of slit images based on respective positions of the plurality of slit images on a detection surface of the imaging element; and determines the height of the top surface of the specimen based on the position of the slit image of the reflected light reflected by the top surface of the specimen on the detection surface.