To provide a locking/unlocking device and an opening device provided with the locking/unlocking device capable of easily locking/unlocking from the indoor side while preventing illegal unlocking from the outside.

This locking/unlocking device is provided with a lock part 6 and a bearing seat. The lock part is provided with a cylindrical member 80 directly or indirectly engaged with the tip 20a of a cylinder lock and the tip of a thumb-turn shaft and rotated by key operation to the cylinder lock 20 or the operation of a thumb-turn 40, and a locking member 7 engaged with and disengaged from the bearing seat interlockingly with the rotation of the cylindrical member to allow locking/unlocking. The cylindrical member and the tip of the thumb-turn shaft are engaged with a play of approximately 90° in a rotating direction. The thumb-turn is provided with the thumb-turn shaft 41, projecting parts 42 mounted orthogonally to the shaft, and rotating plates 61, 62 rotatably mounted onto the same plane as the rotational plane of the projecting parts determined by the rotation of the shaft and having cutout part at the outer peripheral parts. When the rotating plates are rotated to move the positions of the cutout parts, the presence or absence of positional interference between the projecting parts and the rotating plates arises, which regulates the rotation of the thumb-turn shaft or releases the regulation.