To precisely and efficiently perform a wall surface inspection by providing an ultrasonic sensor two-dimensionally arranged to the wall surface, and transmitting and receiving ultrasonic wayes to the wall surface while mechanically scanning in one-dimensional direction.

A screw thread 14 is driven by an actuator 15 while transmitting and receiving ultrasonic waves to an earth wall surface by ultrasonic sensors 16 to vertically move a detecting part 13 having ultrasonic sensors 16 arranged zigzag thereon, and a mechanical scanning is performed every line of the vertically adjacent ultrasonic sensors 16, whereby a detection can be performed with a precision of 1/2 of scanning pitch. The output signal is transmitted to an image processing device, image processed, and displayed. This device is provided on an excavating machine for excavating a continuous wall, and ultrasonic waves are transmitted and received while the ultrasonic sensors 16 are mechanically scanned in onedimensional direction to detect the state the earth wall surface, whereby the earth wall surface in excavating work can be efficiently inspected, the state can be grasped in real time, and a large effect also in safety can be exhibited.