An ultrasonic flaw detector comprising: a flaw detection head including a probe that is configured to transmit an ultrasonic wave to an inspection object formed by a composite member; and a support mechanism disposed such that the support mechanism comes into contact with a lower surface of the inspection object, the support mechanism being configured to support the inspection object, characterized in that the flaw detection head is configured to receive the ultrasonic wave that has reflected on the inspection object, in that the ultrasonic flaw detector comprises a moving mechanism that is configured to cause the flaw detection head to perform scanning, and in that the support mechanism is configured to come into contact with the inspection object and is disposed in a region where flaw detection inspection is performed and the support mechanism includes: a base; and a plurality of protruding portions provided upright on the base and configured such that distal ends of the respective protruding portions come into contact with the lower surface of the inspection object in a region which is subjected to the flaw detection by the probe, and wherein a total number of protruding portions, an arrangement density of the protruding portions and an area of the distal ends of the protruding portions are set such that, over a region of the inspection object, the region being contacted by the support mechanism when the ultrasonic flaw detector performs ultrasonic flaw detection inspection of the inspection object, the variation of the magnitude of the ultrasonic wave that has reflected from the back surface of the inspection object at a position where the support mechanism is in contact with the inspection object and that is received by the probe is within the variation range of the material noise. a flaw detection head including a probe that is configured to transmit an ultrasonic wave to an inspection object formed by a composite member; and a support mechanism disposed such that the support mechanism comes into contact with a lower surface of the inspection object, the support mechanism being configured to support the inspection object, characterized in that the flaw detection head is configured to receive the ultrasonic wave that has reflected on the inspection object, in that the ultrasonic flaw detector comprises a moving mechanism that is configured to cause the flaw detection head to perform scanning, and in that the support mechanism is configured to come into contact with the inspection object and is disposed in a region where flaw detection inspection is performed and the support mechanism includes: a base; and a plurality of protruding portions provided upright on the base and configured such that distal ends of the respective protruding portions come into contact with the lower surface of the inspection object in a region which is subjected to the flaw detection by the probe, and wherein a total number of protruding portions, an arrangement density of the protruding portions and an area of the distal ends of the protruding portions are set such that, over a region of the inspection object, the region being contacted by the support mechanism when the ultrasonic flaw detector performs ultrasonic flaw detection inspection of the inspection object, the variation of the magnitude of the ultrasonic wave that has reflected from the back surface of the inspection object at a position where the support mechanism is in contact with the inspection object and that is received by the probe is within the variation range of the material noise.