Abstract: In a portable electronic device that has a first casing 1 and a second casing 2 coupled to each other via a coupling member and can be selectively set in a closed state and an open state, at an opposing portion between the coupling member and the second casing 2, there are formed a receiving surface 33 and a slide surface 29 that are to be in a slide contact with each other in a first half of a process where both the casings shift from a most separated state to a closest state in the open state, and at an opposing portion between the first casing 1 and the second casing 2, there are formed a projecting portion 10 and a recessed portion 20 that are to be engaged with each other and disengaged from each other in a latter half of the process.