An image forming apparatus comprising: a conveyer configured to convey a sheet; a main body including a main body guide configured to guide the sheet that is conveyed by the conveyer, the main body further including an image forming unit configured to form an image on the sheet; a movable unit being a sheet tray including a movable unit guide configured to guide the sheet that is conveyed by the conveyer, the movable unit being movable with respect to a guide position at which the sheet is guided; wherein: a position detector configured to output a position detection signal according to a state of the movable unit representing whether the movable unit is in the guide position or not; and a controller configured to: determine whether the movable unit is in the guide position based on the position detection signal; perform, if the sheet is in a feeding area and if it is determined that the movable unit is not in the guide position, a feeding execution process in which controlling the conveyer to feed the sheet by a feeding amount and stop the sheet; perform, if the sheet is in the feeding area and if it is determined that the movable unit is in the guide position, a feeding inexecution process in which controlling the conveyer not to feed the sheet by the feeding amount and stop the sheet; and stop feeding the sheet if it is determined that the movable unit is in the guide position based on the position detection signal during feeding of the sheet; and wherein the main body further includes an opposing member being an upper guide arranged along the main body guide and to face the main body guide, the upper guide and the main body guide having a space therebetween and the main body guide and the opposing member guide the sheet in the space therebetween. a conveyer configured to convey a sheet; a main body including a main body guide configured to guide the sheet that is conveyed by the conveyer, the main body further including an image forming unit configured to form an image on the sheet; a movable unit being a sheet tray including a movable unit guide configured to guide the sheet that is conveyed by the conveyer, the movable unit being movable with respect to a guide position at which the sheet is guided; wherein: a position detector configured to output a position detection signal according to a state of the movable unit representing whether the movable unit is in the guide position or not; and a controller configured to: determine whether the movable unit is in the guide position based on the position detection signal; perform, if the sheet is in a feeding area and if it is determined that the movable unit is not in the guide position, a feeding execution process in which controlling the conveyer to feed the sheet by a feeding amount and stop the sheet; perform, if the sheet is in the feeding area and if it is determined that the movable unit is in the guide position, a feeding inexecution process in which controlling the conveyer not to feed the sheet by the feeding amount and stop the sheet; and stop feeding the sheet if it is determined that the movable unit is in the guide position based on the position detection signal during feeding of the sheet; and wherein the main body further includes an opposing member being an upper guide arranged along the main body guide and to face the main body guide, the upper guide and a position detector configured to output a position detection signal according to a state of the movable unit representing whether the movable unit is in the guide position or not; and a controller configured to: determine whether the movable unit is in the guide position based on the position detection signal; perform, if the sheet is in a feeding area and if it is determined that the movable unit is not in the guide position, a feeding execution process in which controlling the conveyer to feed the sheet by a feeding amount and stop the sheet; perform, if the sheet is in the feeding area and if it is determined that the movable unit is in the guide position, a feeding inexecution process in which controlling the conveyer not to feed the sheet by the feeding amount and stop the sheet; and stop feeding the sheet if it is determined that the movable unit is in the guide position based on the position detection signal during feeding of the sheet; and wherein the main body further includes an opposing member being an upper guide arranged along the main body guide and to face the main body guide, the upper guide and determine whether the movable unit is in the guide position based on the position detection signal; perform, if the sheet is in a feeding area and if it is determined that the movable unit is not in the guide position, a feeding execution process in which controlling the conveyer to feed the sheet by a feeding amount and stop the sheet; perform, if the sheet is in the feeding area and if it is determined that the movable unit is in the guide position, a feeding inexecution process in which controlling the conveyer not to feed the sheet by the feeding amount and stop the sheet; and stop feeding the sheet if it is determined that the movable unit is in the guide position based on the position detection signal during feeding of the sheet; and the main body guide having a space therebetween and the main body guide and the opposing member guide the sheet in the space therebetween.