An ultrasonic flaw detection device and an object to be inspected, the ultrasonic flaw detection device comprising: a transmission window; a transmission probe, which is configured to transmit an ultrasonic beam from the transmission window toward a range required to be inspected, which is set in advance, of an inspection surface of the object to be inspected; a reception window, which is arranged in a relationship with respect to the transmission window in which an ultrasonic wave that has been reflected outside the range required to be inspected is inhibited from reaching the reception window; and a reception probe, which is configured to receive an ultrasonic wave that has passed through the reception window, wherein the transmission probe is formed so that an edge of the ultrasonic beam has an angle at which the edge of the ultrasonic beam reaches an edge on the transmission probe side of the range required to be inspected or an inside of the edge on the transmission probe side of the range required to be inspected, wherein the transmission window and the reception window have angles with respect to the inspection surface that establish the relationship in which the ultrasonic wave that has been reflected outside the range required to be inspected is inhibited from reaching the reception window, wherein the transmission probe has a first opposing surface portion that is opposed to the inspection surface, and the transmission window is provided in a side surface portion that is erected at a right angle from the first opposing surface portion, wherein the reception probe has a second opposing surface portion that is opposed to the inspection surface, and the reception window is provided in a side surface portion that is erected at a right angle from the second opposing surface portion, wherein the object to be inspected comprises first and second partitions, the second partition is stacked on the first partition, and the inspection surface is a joint surface between the upper surface of a rib of the first partition and the bottom surface of the second partition, wherein the second partition has groove portions formed between ribs, and each of the transmission probe and the reception probe is configured to be brought into contact with a bottom surface of a respective groove portion, and wherein a rib of the second partition is formed between the groove portions and is configured to be sandwiched between and in contact with the transmission window and the reception window. a transmission window; a transmission probe, which is configured to transmit an ultrasonic beam from the transmission window toward a range required to be inspected, which is set in advance, of an inspection surface of the object to be inspected; a reception window, which is arranged in a relationship with respect to the transmission window in which an ultrasonic wave that has been reflected outside the range required to be inspected is inhibited from reaching the reception window; and a reception probe, which is configured to receive an ultrasonic wave that has passed through the reception window, wherein the transmission probe is formed so that an edge of the ultrasonic beam has an angle at which the edge of the ultrasonic beam reaches an edge on the transmission probe side of the range required to be inspected or an inside of the edge on the transmission probe side of the range required to be inspected, wherein the transmission window and the reception window have angles with respect to the inspection surface that establish the relationship in which the ultrasonic wave that has been reflected outside the range required to be inspected is inhibited from reaching the reception window, wherein the transmission probe has a first opposing surface portion that is opposed to the inspection surface, and the transmission window is provided in a side surface portion that is erected at a right angle from the first opposing surface portion, wherein the reception probe has a second opposing surface portion that is opposed to the inspection surface, and the reception window is provided in a side surface portion that is erected at a right angle from the second opposing surface portion, wherein the object to be inspected comprises first and second partitions, the second partition is stacked on the first partition, and the inspection surface is a joint surface between the upper surface of a rib of the first partition and the bottom surface of the second partition, wherein the second partition has groove portions formed between ribs, and each of the transmission probe and the reception probe is configured to be brought into contact with a bottom surface of a respective groove portion, and wherein a rib of the second partition is formed between the groove portions and is configured to be sandwiched between and in contact with the transmission window and the reception window.