An ultrasonic flaw detection jig includes a base section with which a probe of the ultrasonic inspection apparatus is brought into contact; and a prominence section provided for the base section and inserted into an inspection object hole of an inspection object. The inspection object hole includes a countersunk section connected with a main surface of the inspection object and extending from the main surface while reducing a diameter; and a connection section connecting a bottom of the countersunk section and the rear surface of the inspection object. The prominence section has a conical shape corresponding to the shape of the countersunk section.