a sheet width direction regulating member slidably positionable in accordance with the sheet size of a recording sheet to regulate a sheet width direction;

a sheet conveyance direction regulating member slidably positionable in accordance with the sheet size of the recording sheet to regulate a sheet conveyance direction substantially perpendicular to the sheet width direction;

two levers each having a concavo-convex pattern, interlocking with the sheet width direction regulating member and the sheet conveyance direction regulating member, respectively, and superimposed in a sheet stacking direction to have the same center of rotation and the same locus formed by a leading end of the concavo-convex pattern away from the center of rotation, the concavo-convex pattern of at least one of the levers shaped to prevent, in a particular detection pattern, erroneous detection attributed to positional displacement of the concavo-convex pattern;

a detection sensor including multiple push switches to detect the sheet size of the recording sheet in accordance with a combined concavo-convex pattern formed by the superimposed levers to selectively press the push switches; and

a pattern supplementing device configured to supplement the concavo-convex pattern of the at least one lever at the leading end of the concavo-convex pattern to prevent, in another detection pattern, erroneous detection attributed to the shaping of the concavo-convex pattern for preventing the erroneous detection attributed to the positional displacement of the concavo-convex pattern with respect to the push switches.