An ultrasonic sensor consists of an ultrasonic transducer and a casing. The transducer includes a metal-made housing having an inner space; a vibrating planar member formed in the housing, wherein an outward surface of the vibrating planar member constitutes an outward surface of the housing and an inward surface of the vibrating planar member faces the inner space; and a driving element attached on an inward surface of the vibrating planar member for vibrating the vibrating planar member. This ultrasonic transducer is assembled within the casing and attached to a vehicle via the casing with the outward surface of the vibrating planar member exposed outwardly. Further, a resin film is attached using an adhesive member applied on a rear surface of the film to cover all the outward surface of the vibrating planner member.