An adjusting device for the conveying belt of a planer includes a fundamental plate assembled on the base of a planer. The fundamental plate has its rear end disposed with a rear roller unit connected with the base of the planer and composed of two adjusting plates and a rear roller. The two adjusting plates are respectively bored with longitudinal insert slots preset in number to enable the adjusting plates to be slidably assembled on the fundamental plate. Each adjusting plate has its front end extending outward vertically and forming a positioning wall pivotally fixed thereon with a positioning member and movably fastened on the base of the planer. The adjusting device for the conveying belt of this invention is simple in structure and easy to be assembled, lowering processing and assembling cost.