A tread portion has blocks that are provided with a plurality of sipes having a zigzag part. A tread rubber is formed of short fiber mixed rubber comprising 1.5 to 25 parts by weight of short fibers in 100 parts by weight of rubber component. The sipes comprise a three dimensional sipe in which each wall surface forms bumps and dips whereby the short fibers are three dimensionally arranged. The three dimensional sipe comprises the zigzag part extending from the tread face to a certain depth, while (1) gradually moving towards a direction and then the opposite direction thereto or (2) changing the length of the segments of the zigzag.