To keep the meshing relation between a chain and a sprocket satisfactory and to smoothly move a gate body in an opening/closing device for a sliding door gate, which has the chain on the gate body, drives the sprocket meshed with the chain and opens or closes the gate.

A drive unit 6 is vertical position adjustably provided a guide support 2 holding the gate body, and the drive unit 6 is provided with lower rollers 8, a vertical position adjustable upper roller 9, and the sprocket 10 provided at a position directly below the upper roller 9. The upper abutting face and the lower abutting face of a chain support provided on the guide support side face of the gate body are pinched between the lower rollers 8 and the upper roller 9 of the drive unit 6. The chain is stored in a chain storage groove opened downward on the lower abutting face of the chain support, the upper end of the sprocket 10 is inserted into the chain storage groove to mesh with the chain, and the gate body is moved by rotatively driving the sprocket 10.