To provide a bolt and a nut to perform a firm fastening work even at a limited space.

A nut 10 consists of two partial nuts 11 and 12 arranged in a step-form manner and the outer peripheral top parts 11 and 12a of the partial nuts 11 and 12 are positionally deviated in a direction of rotation centering around a nut axis and besides, a wrench hole 13 is formed in each side of the partial nuts 11 and 12. Thus, with the nut 10 briskly rotated by using a spanner 23 and a bar wrench 24, members, not shown, are fastened against each other between the but 10 and bolt heads 22 of the bolts 20. As a result, a firm fastening work is carried out even at a limited space.