PURPOSE: To prevent an infant from starting to cry by pain when a ball hits him by performing cutting work on foaming polyethylene having a circular or elliptic cross section while moving rotary cutting edges along a curved surface previously projected on the foaming polyethylene in prescribed curvature.

CONSTITUTION: A cutting work table 1 can move in the horizontal or diametrical direction through a guide rail 3a while placing foaming polyethylene 2 being a raw material of a ball. Mountable-demountable rotary cutting edges 4a and 4b are respectively and independently driven by operation of a control board of a built-in computer, and are respectively and independently raised and lowered through a guide rail 3c, and are used by interlocking with the cutting work table, and are arranged so as to move in the horizontal direction along a preset curve. As a result, the foaming polyethylene 2 is shaven out along a curved surface projected in prescribed curvature. Here, the prescribed curvature means curvature of a curved surface corresponding to an external curved surface of the ball.