To obtain an ultrasonic apparatus for measuring a lateral crack depth which can measure a lateral crack depth surely even in the presence of a shearing defect without being influenced by a change in surface shape of a material to be inspected because of the abrasion, etc., and a change of the apparatus with time.

A transmission probe 12 is arranged at a head outer side face 1a of a rail 1 and a reception probe 16 is arranged at a head inner side face 1b of the rail 1 to face the transmission probe 12. The reception probe 16 is an array type probe comprising a plurality of array elements 16-1, 16-2,..., 16-8 aligned in a depthwise direction of a lateral crack. An ultrasonic signal from the transmission probe 12 is passed through the rail 1, and the passed ultrasonic signal is detected at the reception probe 16. A depth of the lateral crack is measured from a distribution of the array elements 16-4 to 16-8 which can detect the ultrasonic signal from the transmission probe 12 and the array elements 16-1 to 16-3 which cannot detect the ultrasonic signal from the transmission probe 12.