A midsole structure for an athletic shoe includes an upper midsole (3) formed of a soft elastic material, a lower midsole (4) disposed under the upper midsole (3) and formed of a soft elastic material, and a wavy plate assembly (5) interposed between the upper midsole (3) and the lower midsoles (4). The wavy plate assembly (5) is comprised of a wavy board (50) and a pair of sidewall members (51) provided discretely from the wavy board (50) and disposed on opposite sides of the wavy board (50). Each of the sidewall members (51) is composed of a wavy bottom-portion (51b) overlapping the wavy board (50) and an upraised portion (51a) extending upwardly from the wavy bottom portion (51b) to face a side surface of the upper midsole (3). Provision of the wavy board (50) and the sidewall members (51) discretely from each other simplifies a layout of a mold for forming the wavy plate assembly (5), thereby reducing a manufacturing cost.