An ultrasonic sensor includes a transceiver block having a transceiver device for transmitting and receiving ultrasonic waves and a circuit board mounted with an electronic circuit for processing ultrasonic signals transmitted and received through the transceiver device. A housing of the ultrasonic sensor has an opening and a communication hole, and the circuit board is stored into the housing through the opening. The transceiver block is attached to the housing. An electric connector extends through the communication hole of the housing to electrically interconnect the transceiver device and the electronic circuit of the circuit board; and a cover member closes the opening of the housing. The communication hole is sealed by a filler material.