To form groove structure consisting of <111> planes for which the amount of etching is controlled by utilizing the anisotropic etching of Si, only by performing anisotropic etching once.

In the groove structure forming method for forming the groove structure consisting of <111> planes by using the anisotropic etching on the monocrystal Si substrate having <100> planes, a mask having such shape that circular-arc shaped or polygonal re-entrant parts consisting of line segments whose angle is larger than 1° and smaller than 44° to the direction of the <111> planes estimated from orientation flat are oppositely and cyclically formed is used as an etching mask for forming the groove structure.