A guide mechanism for a sliding door includes a rail configured to be mounted on the sliding door, a roller carriage configured to move along the rail, and including a roller bracket and a roller configured to be rotatably mounted on the roller bracket, a hinge arm configured to be pivotally connected to a vehicle body, a first shaft configured to pivotally connect the roller carriage to the hinge arm, and a second shaft configured to pivotally connect the hinge arm to the vehicle body.