To manufacture an inner box of a cooker with a wide selection region of a material and with good productivity but without requiring much cost.

In a title inner box wherein there are joined and fixed a plurality of inner box constituent parts such as a main plate 12, an upper plate 13, and a rear plate, L shaped folded sections 22, 28 are formed on outer edges of joint sections 19, 25 of the facing inner box constituent parts, and the folded sections 22, 28 are overlapped and inwardly folded, whereby the inner box is manufactured with assembling and bending of the inner box constituent parts.