To enhance operability and safety, by making an operator easily viewing a monitor screen of reflecting an image of a rear part, and by making the operator accurately determine a situation of the rear part.

This monitor device for confirming a rear side is installed with a monitor 7 in either of the left or right side of a driver seat 8, in a working machine provided with a rear side monitoring camera 6 and the monitor 7 for displaying the image photographed by the rear side monitoring camera 6, and is constituted to display, on a screen of the monitor 7, the image obtained photographedly by the rear side monitoring camera 6, while processing the image to be brought into a mirror image and to be made equivalent to an image viewed by a back mirror.