To detect a microgap, which can be recognized with eyes only after dismantling an automobile, easily by transmitting and receiving an ultrasonic wave using ultrasonic transmitting means and receiving means.

An ultrasonic transmitting means 100 comprises a section 130 for detecting the charged state of an auxiliary power supply being charged form a first power supply section 110, and a section 170 for generating an ultrasonic wave upon receiving an oscillation signal from an ultrasonic oscillating section 160. An ultrasonic receiving means 300 receives an ultrasonic wave from the ultrasonic transmitting means 100, an ultrasonic signal detecting section 340 detects a voltage generated in a Zener diode from a receiving signal, and a control section 385 control the detected voltage. An A/D converting section 360 performs A/D conversion, a second power supply section 450 receives power through a cigar jack, a sound generating section 380 generates a sound signal upon receiving a signal from control section and a liquid crystal display section 480 displays an ultrasonic signal being detected. According to the arrangement, a microgap in an automobile can be found easily.