The present invention realizes an interactive unit attachment structure that enables a projector to have an electronic pen function regardless of the type of projection surface. The attachment structure of the interactive unit, which is provided with a first sensor and a second sensor and which is attached to a wall hanging unit that suspends the projector from the wall such that the interactive unit can slide in a direction projecting from the wall, includes a slot in a slot provided in the wall hanging unit that extends in the direction of sliding and first and second protrusions provided in the interactive unit that protrude into the slot.