An ultrasonic treatment device includes a first vibration transmitting member and a second vibration transmitting member and being configured to treat a biological tissue by means of vibration transmitted by the first vibration transmitting member and the second vibration transmitting member. At a boundary position between the first vibration transmitting member and the second vibration transmitting member, the first vibration transmitting member has a boundary face including a contacting area that is in contact with the second vibration transmitting member at the face, and a non-contacting area that is not in contact with the second vibration transmitting member.