A door opening and closing device for a slide door, which can be horizontally open and close the slide door by application of a light load and which can be constructed and maintained at low costs. The door opening and closing device has a transmission mechanism for converting through a lever (31) the amount of settlement of a tread plate (21) placed on floor surfaces at positions in front and back of the slide door (12), into a predetermined displacement amount. The tread plate (21) is weight-wise balanced such that the transmission mechanism and settlement of the tread plate due to the weight of an adjustment weight (W) causes the tread plate (21) to float through the lever (31). Up-down movement of the long transmission member (32) presses a drive rotation body (42) to an open door rail (41), and a component force of the pressing force allows the slide door (12) to be opened by application of a light load.