A holding member of a sheet feeder includes a locking member engageable with a part of a pinion gear and holds the pinion gear and a sensor member. The holding member is detachably attachable to the sheet storage portion. The pinion gear and a pair of rack members are meshed with the holding member attached to a sheet storage portion. The holding member holds the pinion gear movably in an axial direction of the rotary shaft so that the pinion gear is displaceable between a first position where the rotation of the pinion gear is regulated and a second position where the pinion gear is rotatable. The pinion gear is located at the first position with the holding member detached from the sheet storage portion and located at the second position with the holding member attached to the sheet storage portion.