To manufacture an adjustable spanner with a usual open engagement part such as a conventional spanner as economically as possible.

An adjustable spanner has a first pawl 12 to be engaged with one side of a hexagonal head and a second pawl 14 to be engaged with the opposite side, both connected in a rotationally movable manner. A cam means is provided between both pawls to operate in the direction of closing between both pawls and an oscillating lever 20 is integrally mounted on the cam means. A stopper 12c and a push spring 18 are provided between the first pawl 12 and the second pawl 14 to restrict the minimum space therebetween and to energize both pawls toward the stopper 12c, respectively. With the oscillation of the oscillating lever 20, a bolt and a nut are fastened or loosened.