An ultrasonic flaw detector for a composite material constituted by a plurality of materials which are different in physical properties from one another includes: a section specifier configured to specify first and second sections of an RF signal of a reflected wave of an ultrasonic wave with which the composite material is irradiated, a material reflected wave being possibly generated in the first section, an interface reflected wave being possibly generated in the second section; a material flaw determiner configured to determine whether or not a value of the RF signal exceeds a positive first threshold in the first section; and an interface flaw determiner configured to determine whether or not the value of the RF signal falls below a negative second threshold in the second section.