An encoder comprising: a slit array (SA1, SA2, SI; SA, SI) comprising a plurality of reflection slits arranged side by side along a first measurement axis; and an optical module capable of relatively moving on said first measurement axis with respect to said slit array (SA1, SA2, SI; SA, SI) while facing part of said slit array (SA1, SA2, SI; SA, SI); wherein said optical module comprises: a point light source that irradiates the part of said slit array (SA1, SA2, SI; SA, SI) with light; and a light receiving array (PA1, PA2; PA) comprising a plurality of light receiving elements (P0, P1, P2, P3, P4) arranged in a plane parallel to said slit array (SA1, SA2, SI; SA, SI) side by side along a second measurement axis (C') corresponding to said first measurement axis but in said plane parallel to said slit array (SA1, SA2, SI; SA, SI) and arranged in positions offset with respect to said point light source in a width direction perpendicular to said second measurement axis (C') in said plane parallel to said slit array (SA1, SA2, SI; SA, SI), the light receiving elements (P0, P1, P2, P3, P4) being respectively receiving light irradiated from said point light source and reflected from said reflection slits, and said plurality of light receiving elements (P0, P1, P2, P3, P4) has respective shapes such that the light receiving element nearer to said point light source in said second measurement axis direction (C') has a shorter length in said width direction than the light receiving element farther away from said point light source in said second measurement axis direction (C'), and such that end parts (En) of the light receiving elements (P0, P1, P2, P3, P4) on an opposite side with respect to said point light source in said width direction R are arranged side by side in positions on a line (Lcp) along said second measurement axis (C'). a slit array (SA1, SA2, SI; SA, SI) comprising a plurality of reflection slits arranged side by side along a first measurement axis; and an optical module capable of relatively moving on said first measurement axis with respect to said slit array (SA1, SA2, SI; SA, SI) while facing part of said slit array (SA1, SA2, SI; SA, SI); wherein said optical module comprises: a point light source that irradiates the part of said slit array (SA1, SA2, SI; SA, SI) with light; and a light receiving array (PA1, PA2; PA) comprising a plurality of light receiving elements (P0, P1, P2, P3, P4) arranged in a plane parallel to said slit array (SA1, SA2, SI; SA, SI) side by side along a second measurement axis (C') corresponding to said first measurement axis but in said plane parallel to said slit array (SA1, SA2, SI; SA, SI) and arranged in positions offset with respect to said point light source in a width direction perpendicular to said second measurement axis (C') in said plane parallel to said slit array (SA1, SA2, SI; SA, SI), the light receiving elements (P0, P1, P2, P3, P4) being respectively receiving light irradiated from said point light source and reflected from said reflection slits, and said plurality of light receiving elements (P0, P1, P2, P3, P4) has respective shapes such that the light receiving element nearer to said point light source in said second measurement axis direction (C') has a shorter length in said width direction than the light receiving element farther away from said point light source in said second measurement axis direction (C'), and such that end parts (En) of the light receiving elements (P0, P1, P2, P3, P4) on an opposite side with respect to said point light source in said width direction R are arranged side by side in positions on a line (Lcp) along said second measurement axis (C'). a point light source that irradiates the part of said slit array (SA1, SA2, SI; SA, SI) with light; and a light receiving array (PA1, PA2; PA) comprising a plurality of light receiving elements (P0, P1, P2, P3, P4) arranged in a plane parallel to said slit array (SA1, SA2, SI; SA, SI) side by side along a second measurement axis (C') corresponding to said first measurement axis but in said plane parallel to said slit array (SA1, SA2, SI; SA, SI) and arranged in positions offset with respect to said point light source in a width direction perpendicular to said second measurement axis (C') in said plane parallel to said slit array (SA1, SA2, SI; SA, SI), the light receiving elements (P0, P1, P2, P3, P4) being respectively receiving light irradiated from said point light source and reflected from said reflection slits, and said plurality of light receiving elements (P0, P1, P2, P3, P4) has respective shapes such that the light receiving element nearer to said point light source in said second measurement axis direction (C') has a shorter length in said width direction than the light receiving element farther away from said point light source in said second measurement axis direction (C'), and such that end parts (En) of the light receiving elements (P0, P1, P2, P3, P4) on an opposite side with respect to said point light source in said width direction R are arranged side by side in positions on a line (Lcp) along said second measurement axis (C').