One oscillator of a dual type ultrasonic probe generates an ultrasonic pulse and receives a surface reflection wave and a boundary face reflection wave from a coated plate. Another oscillator receives a bottom reflection wave from the coated plate. As a result, these reflection waves can be received at high levels, and reception times of the boundary face reflection wave and the bottom reflection wave can be accurately discriminated. Therefore, the thickness of the plate alone can be accurately calculated from the difference in reception times.