A blade has an overall blade width of W, and includes a central front portion, interposition front portions, and side front portions. A straight first edge section is arranged on the lower end of the central front portion. Each of the front surfaces of the central front portion, the interposition front portions, and the side front portions is formed in an arc-shaped surface that has a prescribed radius R2. The radius R2 satisfies the following formula: R2=(0.7 to 1.0).times.H, where H is the height from the edge of the first edge section to the top end of a soil retaining plate in side view in a digging posture when the edge angle .alpha. of the edge sections is an angle of 40.degree. to 55.degree..