A slide door apparatus includes a body-side guide rail provided at a vehicle body, a door-side guide rail provided at a slide door, connection members each of which includes a guide roller rolling on a path formed by each of the guide rails, each of the connection members being connected to each of the guide rails, the slide door being configured to open and close a door opening portion formed at the vehicle body based on a movement of the slide door, facing portions formed at the slide door and the vehicle body respectively, the facing portions facing each other in a state where the slide door is in a fully open position, and a widthwise displacement restraint mechanism restricting a relative movement between the facing portions of the slide door and the vehicle body in a width direction of the vehicle by connecting the facing portions to each other.