There has heretofore been known a device for detecting nearby objects existing around a vehicle.  U.S. Pat.  No.  6,318,774 (International Patent Publication No.  2001-527480A) discloses a device, wherein a bumper of a vehicle is perforated, and an ultrasonic sensor is inserted in the holes so that the head portion of the sensor is in flush with the outer surface of the bumper.  With this device, however, the ultrasonic sensor is exposed to the exterior of the vehicle impairing appearance of the vehicle. 
To cope with this problem, JP-A-10-123236 discloses a device, wherein a recessed portion is formed in the back side of a bumper of a vehicle, and an ultrasonic sensor is fitted into the recessed portion without permitting the device itself to be seen from the outer side to favorably maintain appearance of the vehicle.  According to this device, however, the ultrasonic sensor operates while being fitted into the recessed portion on the back side of the bumper of the vehicle.  Namely, the ultrasonic sensor transmits and receives ultrasonic waves through a wall of the bumper of the vehicle and, hence, propagation efficiency of ultrasonic waves drops and the sensitivity decreases.