An ultrasonic flaw detector comprising: an ultrasonic probe comprising a plurality of ultrasonic elements and configured to emit an ultrasonic wave on an object to be inspected and to receive a reflected ultrasonic wave from the object; a drive element control unit configured to control the plurality of ultrasonic elements to emit the ultrasonic wave on the object and to control a reflected ultrasonic wave from the object; a delay control unit configured to control an oscillation of each of the plurality of ultrasonic elements with an appropriate delay time; and an analysis unit configured to analyse received signal of the reflected ultrasonic wave and to calculate a flaw detection result, a calculation unit configured to obtain an incident angle (α) of the ultrasonic wave entering an incident position by using a refraction angle (β) of the ultrasonic wave incident in the object, the incident position of the ultrasonic wave on a surface of the object and a surface shape of a surface of the object at the incident position, and to back calculate the center position (Ec) of a plurality of simultaneously driven ultrasonic elements, based on the incident position and the incident angle (α) by maintaining uniform refraction angles (β) and making incident points and focal points on the object constants, wherein the calculation unit is further configured to calculate and to obtain the incident position based on the refraction angle (β) and a focal point of the ultrasonic wave in a case of driving the ultrasonic element under flaw detection conditions for being used for the object having a planar surface shape. an ultrasonic probe comprising a plurality of ultrasonic elements and configured to emit an ultrasonic wave on an object to be inspected and to receive a reflected ultrasonic wave from the object; a drive element control unit configured to control the plurality of ultrasonic elements to emit the ultrasonic wave on the object and to control a reflected ultrasonic wave from the object; a delay control unit configured to control an oscillation of each of the plurality of ultrasonic elements with an appropriate delay time; and an analysis unit configured to analyse received signal of the reflected ultrasonic wave and to calculate a flaw detection result, a calculation unit configured to obtain an incident angle (α) of the ultrasonic wave entering an incident position by using a refraction angle (β) of the ultrasonic wave incident in the object, the incident position of the ultrasonic wave on a surface of the object and a surface shape of a surface of the object at the incident position, and to back calculate the center position (Ec) of a plurality of simultaneously driven ultrasonic elements, based on the incident position and the incident angle (α) by maintaining uniform refraction angles (β) and making incident points and focal points on the object constants, wherein the calculation unit is further configured to calculate and to obtain the incident position based on the refraction angle (β) and a focal point of the ultrasonic wave in a case of driving the ultrasonic element under flaw detection conditions for being used for the object having a planar surface shape.