An installation device includes a retaining member fixed to a rail of the slide assembly and including a base, a release portion, an arm and a resilient portion. The base contacts the first contact portion of the rail. The release portion extends from the base. The arm has a retaining hole and a positioning portion. The retaining hole is located corresponding to an installation hole of the rail, and the positioning portion contacts the second contact portion of the rail. The resilient portion is connected to the rail so as to provide a resilient force to keep the retaining hole of the arm facing the installation hole of the rail. When a force is applied to the release portion, the arm is moved and the positioning portion is disengaged from the second contact portion, so that the installation member is released from the installation hole of the rail.