A portable electronic device includes an expansion base module, a pair of sliding members mounted slidably on the expansion base module, a system module connected to the sliding members, and a synchronizing unit including a pair of first front and rear pulleys, a pair of second front and rear pulleys, and an endless cord that is connected to the sliding members and that trains on the first front and rear and second front and rear pulleys in such a manner to permit synchronous sliding movements of the sliding members.