A securing device for a portable device has a lock assembly and a cable connected to the lock assembly. The lock assembly has two locking pins, two resilient elements, a slider, and a lock mounted in a housing. When the lock is rotated by a key, the lock drives the slider and the slider pushes the locking pins to slide laterally. Thus, the lock assembly can be fastened to a casing with a lock hole and with interior space laterally reserved beside the lock hole. The securing device secures the portable device regardless of thickness of the portable device.