A midsole includes a thick plate-shaped or column-shaped cushioning portion. A plurality of grooves is formed on an outer peripheral surface of the cushioning portion. The respective grooves are helically formed around a substantially vertical line. The respective grooves are arranged substantially parallel with each other. A range .alpha. in which each of the grooves is formed is larger than a range of 15 degrees around the axial line and is smaller than a range of 180 degrees around the axial line.