An ultrasonic flaw detection device comprising: a flaw detection mechanism that inspects an object-to-be-inspected by transmitting and receiving ultrasonic waves; a movement mechanism that moves a position of the flaw detection mechanism with respect to the object-to-be-inspected; and a control unit that controls the flaw detection mechanism and the movement mechanism, wherein the flaw detection mechanism includes a flaw detection unit having a probe that transmits and receives the ultrasonic waves and a contact part on which a contact surface that comes into contact with a surface-to-be-inspected of the object-to-be-inspected is formed, a swinging mechanism (23, 24) that supports the flaw detection unit so as to be swingable around a swinging axis, and a lock mechanism (25, 26) that switches between a swinging state where the flaw detection unit can swing around the swinging axis and a locked state where the flaw detection unit cannot swing around the swinging axis, and the control unit controls the lock mechanism and the movement mechanism so as to cause the flaw detection unit to be in the locked state in a state where a tangential direction to the contact surface and a tangential direction to the surface-to-be-inspected coincide with each other, when moving the flaw detection unit from a position separated from a first end portion of the surface-to-be-inspected to a lock release position (Prl) in the vicinity of the first end portion along the tangential direction to the surface-to-be-inspected, and switch the locked state to the swinging state after the flaw detection unit has reached the lock release position. a flaw detection mechanism that inspects an object-to-be-inspected by transmitting and receiving ultrasonic waves; a movement mechanism that moves a position of the flaw detection mechanism with respect to the object-to-be-inspected; and a control unit that controls the flaw detection mechanism and the movement mechanism, wherein the flaw detection mechanism includes a flaw detection unit having a probe that transmits and receives the ultrasonic waves and a contact part on which a contact surface that comes into contact with a surface-to-be-inspected of the object-to-be-inspected is formed, a swinging mechanism (23, 24) that supports the flaw detection unit so as to be swingable around a swinging axis, and a lock mechanism (25, 26) that switches between a swinging state where the flaw detection unit can swing around the swinging axis and a locked state where the flaw detection unit cannot swing around the swinging axis, and the control unit controls the lock mechanism and the movement mechanism so as to cause the flaw detection unit to be in the locked state in a state where a tangential direction to the contact surface and a tangential direction to the surface-to-be-inspected coincide with each other, when moving the flaw detection unit from a position separated from a first end portion of the surface-to-be-inspected to a lock release position (Prl) in the vicinity of the first end portion along the tangential direction to the surface-to-be-inspected, and switch the locked state to the swinging state after the flaw detection unit has reached the lock release position.