An ultrasonic flaw detection device for detecting a planar defect present at a butted portion of a groove in a welded portion of a welded steel pipe, the ultrasonic flaw detection device comprising: a wedge formed as a polyhedron having an installation surface where a matrix array probe is installed, the matrix array probe that is configured to be arranged, via the wedge, at a position on an outer peripheral surface of the welded steel pipe, the position enabling ultrasonic waves to be vertically incident on a welding root surface, the matrix array probe configured to transmit ultrasonic waves focused in a pipe axis direction to the welding root surface, and to receive ultrasonic waves reflected at the welding root surface; an ultrasonic wave transmitting and receiving unit configured to control the matrix array probe such that the matrix array probe receives the reflected ultrasonic waves reflected at the welding root surface; and an evaluation unit configured to detect the planar defect based on the reflected ultrasonic waves received by the ultrasonic wave transmitting and receiving unit, wherein the matrix array probe has a plurality of vibration elements arranged lattice-like, an array pitch of the vibration elements in the pipe axis direction is larger than a wavelength of ultrasonic waves transmitted and received, widths of the vibration elements in the pipe axis direction decrease outward in the pipe axis direction from a pipe axis direction center position of the matrix array probe, and the widths and center coordinates of the vibration elements in the pipe axis direction are set so that all of ultrasonic waves from the vibration elements can overlap in a focal position control range for the ultrasonic waves, wherein the widths and center coordinates of the vibration elements in the pipe axis direction are set so as to satisfy the following Equation: φ n ≥ θ n where the directivity angle φ n of the n-th vibration element is calculated by the following Equation: φ n = 29 λ Ew n and the angle θ n of the n-th vibration element is calculated by the following Equation: θ n = tan − 1 abs X n − X c f min where λ is the wavelength of ultrasonic waves, X n is the center coordinate of the n-th vibration element, Ew n is the width of the n-th vibration element, X c is the center position of the matrix array probe, and f min is the focal distance. a wedge formed as a polyhedron having an installation surface where a matrix array probe is installed, the matrix array probe that is configured to be arranged, via the wedge, at a position on an outer peripheral surface of the welded steel pipe, the position enabling ultrasonic waves to be vertically incident on a welding root surface, the matrix array probe configured to transmit ultrasonic waves focused in a pipe axis direction to the welding root surface, and to receive ultrasonic waves reflected at the welding root surface; an ultrasonic wave transmitting and receiving unit configured to control the matrix array probe such that the matrix array probe receives the reflected ultrasonic waves reflected at the welding root surface; and an evaluation unit configured to detect the planar defect based on the reflected ultrasonic waves received by the ultrasonic wave transmitting and receiving unit, wherein the matrix array probe has a plurality of vibration elements arranged lattice-like, an array pitch of the vibration elements in the pipe axis direction is larger than a wavelength of ultrasonic waves transmitted and received, widths of the vibration elements in the pipe axis direction decrease outward in the pipe axis direction from a pipe axis direction center position of the matrix array probe, and the widths and center coordinates of the vibration elements in the pipe axis direction are set so that all of ultrasonic waves from the vibration elements can overlap in a focal position control range for the ultrasonic waves, wherein the widths and center coordinates of the vibration elements in the pipe axis direction are set so as to satisfy the following Equation: φ n ≥ θ n where the directivity angle φ n of the n-th vibration element is calculated by the following Equation: φ n = 29 λ Ew n and the angle θ n of the n-th vibration element is calculated by the following Equation: θ n = tan − 1 abs X n − X c f min where λ is the wavelength of ultrasonic waves, X n is the center coordinate of the n-th vibration element, Ew n is the width of the n-th vibration element, X c is the center position of the matrix array probe, and f min is the focal distance.