PURPOSE: To decide accurately properties of a sample plate body by providing a delay circuit between each oscillation detecting coil and a discriminator and inputting plate waves to the discriminator as (a) or (s)-mode plate waves regardless of propagation of intermediate-mode plate waves.

CONSTITUTION: If a phase difference is generated between outputs of detecting coils 26a and 26b because the relative position relation between oscillation detecting coils 26a and 26b is shifted in the lengthwise direction from the original relation, a signal Sa is outputted from an amplifier 28a, and a signal Sb is outputted from an amplifier 28b. When they are converted to the same phase through delay circuits 29a and 29b and the output is taken out from an adder 30, they can be inputted to a discriminator 14 as plate waves Se of an optional single mode of (a) mode or S mode, and thus, properties of the sample plate body are decided accurately, and simultaneously the S/N is improved.