A portable electronic apparatus includes a casing including a base and an upper cover, and an electronic processing module disposed in the base. The base includes a first engagement structure having a lock slot. The upper cover includes a plate member and a second engagement structure mounted on the plate member. The second engagement structure is presented as a cantilever structure and includes a locking part and a detaching part connected to the locking part. The base and the upper cover are engaged such that the locking part is locked in the lock slot. The detaching part can be moved so as to detach the locking part from the lock slot. Therefore, a user can detach the locking part from the lock slot by the detaching part without any tools, which greatly reduces the complexity of the detachment of a casing of a conventional portable electronic apparatus.