PURPOSE: To establish a gate control system suitable for an ultrasonic investigation into an inclined cross section or a curved surface-shaped cross section by arranging modes to measure so as to obtain an investigation image on an ultrasonic wave reflecting surface such as a different kind interface or a contact plane situated inside of a work in the whole measurement range area.

CONSTITUTION: A sampling program stored in a ROM 15 has two operation modes corresponding to an ordinary mode and a specific mode. The specific mode is to obtain an ultrasonic investigation image by using an ultrasonic wave reflecting surface as a measurement object surface when the ultrasonic wave reflecting surface such as a different kind interface or a contact plane exists inside of a work over the whole range area to be measured. A counter circuit is provided in an ultrasonic transmitter-receiver 5 so as to determine gate delay time in gate control according to a preset counter value, and a new count value can be reset in this circuit by program control. Thereby, the gate delay time in the gate control can be set and changed.