A door apparatus includes a door having a hole and configured to be slidable with respect to a vehicle body, a member configured to open and close the hole, a first detector configured to detect a position of the member relative to the hole to determine an openness of the hole, a second detector configured to detect a position of the door relative to the body, and a unit configured to be activated to project from one to another of the body and the door, so as to abut on the another such that the door is stopped from sliding in an opening direction, when the openness is greater than a threshold, and the position is between a full-closed position and a midway position.