To provide a component size adjustment mechanism capable of easily adjusting size of a component in a predetermined direction, a size adjusting device, and a component having a size adjustment mechanism.

A formwork support metal fitting 10 comprises: a support bar component 13 which is vertically installed on each of a pair of support base plates 11, and a formwork support 25 which is supported by the both support bar components 13. The support bar component 13 is divided into two at a separation position 14c located midway of an intermediate bar part 14, such that a size adjusting device 18 is attached therebetween. The size adjusting device 18 is provided with first and second nut components 19 and 21 which are fastened to the intermediate bar part 14 at the both sides of the separation position and a bolt component 23. The first and second nut components 19 and 21 are provided with a right-handed screw hole 19a and a left-handed screw hole 21a which are inversely threaded to each other. The bolt component 23 is provided with a right-hand screw 23a on the one side in the longitudinal direction, a left-hand screw 23b on the other side, and an engaging groove 24 at each end for engagement with the head of a screw driver 2. The right-handed screw 23a is screwed into the right-handed screw hole 19a, while the left-handed screw 23b is screwed into the left-handed screw hole 21a.