An encoder comprising: a disc having a circular surface with a central axis and being rotatable around the central axis, the disc having a slit row provided on the circular surface, the slit row including slits arranged in a circumferential direction of the circular surface around the central axis and in a radial direction of the circular surface; and a sensor provided opposite to the slit row and comprising: a first light receiver (PIL1, PIL2) configured to output a first light receiving signal as the slit row rotates along the circumferential direction when the disc rotates around the central axis; and a second light receiver configured to output a second light receiving signal as the slit row moves along the radial direction when the disc rotates around the central axis; wherein the first light receiver (PILI, PIL2) and the second light receiver are arranged symmetrically with respect to a symmetry axis (Lr) which passes an optical axis (Op) of a light source along the radial direction; wherein: the first light receiver (PILI, PIL2) includes first light receiving elements arranged in the circumferential direction and configured to receive light transmitting through or reflected at the slit row and to output the first light receiving signal, and the second light receiver includes second light receiving elements arranged in the radial direction and configured to receive light transmitting through or reflected at the slit row and to output the second light receiving signal; wherein the first light receiver (PIL1, PIL2) and the second light receiver are arranged in the circumferential direction; wherein the first light receiver (PIL1, PIL2) includes a first light receiving array and a second light receiving array which are arranged separately in the circumferential direction, and the second light receiver is positioned between the first light receiving array (PILI) and the second light receiving array. a disc having a circular surface with a central axis and being rotatable around the central axis, the disc having a slit row provided on the circular surface, the slit row including slits arranged in a circumferential direction of the circular surface around the central axis and in a radial direction of the circular surface; and a sensor provided opposite to the slit row and comprising: a first light receiver (PIL1, PIL2) configured to output a first light receiving signal as the slit row rotates along the circumferential direction when the disc rotates around the central axis; and a second light receiver configured to output a second light receiving signal as the slit row moves along the radial direction when the disc rotates around the central axis; a first light receiver (PIL1, PIL2) configured to output a first light receiving signal as the slit row rotates along the circumferential direction when the disc rotates around the central axis; and a second light receiver configured to output a second light receiving signal as the slit row moves along the radial direction when the disc rotates around the central axis; wherein the first light receiver (PILI, PIL2) and the second light receiver are arranged symmetrically with respect to a symmetry axis (Lr) which passes an optical axis (Op) of a light source along the radial direction; wherein: the first light receiver (PILI, PIL2) includes first light receiving elements arranged in the circumferential direction and configured to receive light transmitting through or reflected at the slit row and to output the first light receiving signal, and the second light receiver includes second light receiving elements arranged in the radial direction and configured to receive light transmitting through or reflected at the slit row and to output the second light receiving signal; wherein the first light receiver (PIL1, PIL2) and the second light receiver are arranged in the circumferential direction; wherein the first light receiver (PIL1, PIL2) includes a first light receiving array and a second light receiving array which are arranged separately in the circumferential direction, and the second light receiver is positioned between the first light receiving array (PILI) and the second light receiving array. the first light receiver (PILI, PIL2) includes first light receiving elements arranged in the circumferential direction and configured to receive light transmitting through or reflected at the slit row and to output the first light receiving signal, and the second light receiver includes second light receiving elements arranged in the radial direction and configured to receive light transmitting through or reflected at the slit row and to output the second light receiving signal; wherein the first light receiver (PIL1, PIL2) and the second light receiver are arranged in the circumferential direction; wherein the first light receiver (PIL1, PIL2) includes a first light receiving array and a second light receiving array which are arranged separately in the circumferential direction, and the second light receiver is positioned between the first light receiving array (PILI) and the second light receiving array.