In a portable telephone, the front side of a back casing is covered with a front casing. A circuit substrate is housed in a housing concave portion of the back casing and the front side of the circuit substrate is covered with a waterproof type keypad sheet material. The outer edge section of the waterproof type keypad sheet material is fastened between the outer edge section of the back casing and the outer edge section of the front casing. A mounting hole is formed in the waterproof type keypad sheet material and a microphone holder is inserted into the mounting hole. A microphone is mounted in the microphone holder, a waterproof membrane is affixed on the end face of the microphone holder and a small opening of the microphone holder is blocked by the waterproof membrane.