PURPOSE: To enable to evaluate the bonded surfaces quantitatively and accurately, with the contact state stable for the prober and bonded member, by radiating ultrasonic waves on the bonded planes of metal material from a flat plane orthogonal to them.

CONSTITUTION: Bonded members 1a, 1b are bonded at a bonded plane 2, ultrasonic probers 3 and 4 to find the bonded rate are incorporated, and located on the orthogonal plane 5 to the bonded plane 2 by clipping the plane 2 and in parallel with the plane 2 with a given interval. The reception of ultrasonic waves to the probers 3, 4 is made with a crack-finder 6. In this case, after the ultrasonic waves from the prober 3 are reflected on the bottom 10 of the bonded object 1a, they are received at the prober 4 through the bonded plane 2. Further, the reception level of the prober 4 is different depending on the transmission or reflection of the ultrasonic waves at the bonded part 11 and the notbonded part 12 as defect, allowing to make easy the judgement of the bonded rate.