A portable memory device (1) includes a memory unit, a USB mating port (20) electrically connecting with the memory unit, a protective case (30) detachably mated with the USB mating port and a plastic waterproof member (32) secured in the case. The plastic waterproof member is pressed against a periphery (213) of the USB mating port to seal a gap between the USB mating port and the case. The USB mating port is protected in the case form water penetration. Moreover, the plastic waterproof member imposes a force rising from plastic deformation of the plastic waterproof member on the USB mating port. Therefore, the USB mating port and the case is firmly engaged with each other.