A root canal treating device includes a root canal treating hand piece 50 having, attached thereto, a visible light camera 60 that captures a 2D captured image P of a tooth T as a target of interest； a position detection unit 80 that detects a position of the root canal treating hand piece 50； a monitor 15 that displays an visible light image； and an image generation unit 21 that displays, in an overlapping manner on the monitor 15, a converted horizontal cross-sectional image Cs1 based on 3D information including information on a root canal R inside the tooth T and the 2D captured image captured by the visible light camera 60.
