To provide an opening device having a door-opening auxiliary mechanism of a simple structure capable of obtaining a big opening force without requiring a complicated operation for opening a door.

The opening device includes a frame body mounted along the four sides of an opening of a building and a door arranged inside of the frame body and opening inward or outward from the room. Furthermore, the device includes the following: a latch bolt projectable/retractable from an end face where a rotary axis of the door is not provided and engageable/disengageable to the frame body; a latch bolt control mechanism 40 provided on the door for prohibiting or permitting the latch bolt projecting/retracting from the door; and an assisting mechanism 60 provided on the door and working with the latch bolt control mechanism for helping the door, in the state wherein the latch bolt is permitted for projecting/retracting from the door, and energized in the direction to detach from the frame body.