A sheet feeder comprising: a feed tray configured to support a sheet placed thereon; a pickup roller configured to rotate around a pickup roller axis and feed the sheet placed on the feed tray in a sheet feeding direction; a stopper movable between: a first position where the stopper is close to the pickup roller; and a second position where the stopper is farther from the pickup roller than when the stopper is in the first position; a driving mechanism configured to move the stopper between the first position and the second position; and a set guide having a first surface (431H, 432H) facing in a particular direction such as to become farther away from a surface (411A, 412A) of the pickup roller, the set guide being rotatable around a first rotational axis (431C, 432C) along the pickup roller axis, the set guide being movable between: a third position where the first surface (431H, 432H) is positioned downstream of all the surface (411A, 412A) of the pickup roller in the particular direction; and a fourth position where the first surface (431H, 432H) is positioned upstream of at least a part of the surface (411A, 412A) of the pickup roller in the particular direction, characterized in that the first rotational axis (431C, 432C) is positioned at a middle portion of the set guide in the sheet feeding direction, wherein, in response to the stopper being placed in the first position by the driving mechanism, the stopper comes into contact with an end portion (431E, 432E) of the set guide, and the set guide is placed in the third position, and wherein, in response to the stopper being placed in the second position by the driving mechanism, the stopper is separated away from the end portion (431E, 432E) of the set guide, and the set guide is placed in the fourth position. a feed tray configured to support a sheet placed thereon; a pickup roller configured to rotate around a pickup roller axis and feed the sheet placed on the feed tray in a sheet feeding direction; a stopper movable between: a first position where the stopper is close to the pickup roller; and a second position where the stopper is farther from the pickup roller than when the stopper is in the first position; a first position where the stopper is close to the pickup roller; and a second position where the stopper is farther from the pickup roller than when the stopper is in the first position; a driving mechanism configured to move the stopper between the first position and the second position; and a set guide having a first surface (431H, 432H) facing in a particular direction such as to become farther away from a surface (411A, 412A) of the pickup roller, the set guide being rotatable around a first rotational axis (431C, 432C) along the pickup roller axis, the set guide being movable between: a third position where the first surface (431H, 432H) is positioned downstream of all the surface (411A, 412A) of the pickup roller in the particular direction; and a fourth position where the first surface (431H, 432H) is positioned upstream of at least a part of the surface (411A, 412A) of the pickup roller in the particular direction, a third position where the first surface (431H, 432H) is positioned downstream of all the surface (411A, 412A) of the pickup roller in the particular direction; and a fourth position where the first surface (431H, 432H) is positioned upstream of at least a part of the surface (411A, 412A) of the pickup roller in the particular direction, characterized in that the first rotational axis (431C, 432C) is positioned at a middle portion of the set guide in the sheet feeding direction, wherein, in response to the stopper being placed in the first position by the driving mechanism, the stopper comes into contact with an end portion (431E, 432E) of the set guide, and the set guide is placed in the third position, and wherein, in response to the stopper being placed in the second position by the driving mechanism, the stopper is separated away from the end portion (431E, 432E) of the set guide, and the set guide is placed in the fourth position.