A sheet tray device including (a) a first tray and (b) a second tray which is disposed on an upper side of the first tray and which includes (b-1) a support member which bridges between opposite side walls of the first tray and which is movable relative to the opposite side walls, and (b-2) a supported body pivotably supported by the support member. Each of the opposite side walls includes a first rail portion and a second rail portion that is located on an upper side of the first rail portion. The support member includes (i) a main slide portion that is slidably held in contact with an upper surface of the first rail portion, (ii) a removal-preventing slide portion that extends through a space between the first and second rail portions, and (iii) a rotation-preventing slide portion that is slidably held in contact with a side surface of the first rail portion. Also disclosed is an image forming apparatus including the sheet tray device.