A substrate transfer apparatus comprising: a plurality of floating-transfer guide plates adjacent to each other with a space, each of guide plate having a plurality of floating gas ejecting holes; a gas supplying source for supplying a floating gas to the guide plates; a tray that is placed on one of the guide plates in order to mount a substrate to be transferred, and that is floated by the floating gas; and a transfer arm for transferring the floated tray to the adjacent other guide plate from the guide plate, wherein the tray includes a main body portion having both side edges parallel to a transfer direction of the tray, and an outward projecting portion that is formed so as to partially project outwardly from at least one of both side edges of the main body portion, and wherein the transfer arm is in contact and engaged with the outward projecting portion when the tray is transferred by the transfer arm.