An end effector comprising: an attachment plate; a hand including a proximal end portion fixed to one end of the attachment plate, a pair of distal end portions (10b, 10c) branching off from the proximal end portion, and a slit formed between the pair of distal end portions (10b, 10c) and the proximal end portion; a holder provided on the hand and configured to hold a first substrate; a mapping detector provided at the pair of distal end portions of the hand (10b, 10c), the mapping detector being configured to face a second substrate stored in a cassette and detect presence or absence of the second substrate; a light emitter, which is built-in in the attachment plate and which is configured to convert an electrical input to generate detection light; a light receiver, which is provided on an upper surface of the attachment plate and which is configured to receive the detection light and convert the detection light into an electrical output; and an optical path formed in the hand, through which the detection light emitted from the light emitter passes to be incident on the light receiver, wherein the optical path is formed such that the detection light is blocked by the first substrate held by the holder, and such that the detection light is blocked by the second substrate detected by the mapping detector, the optical path includes: a first light guiding section extending from the light emitter to a lower surface of one of the distal end portions of the hand; a second light guiding section extending from an end of the first light guiding section to a lower surface of the other distal end portion of the hand; and a third light guiding section extending from an end of the second light guiding section through the slit to the light receiver, part of the third light guiding section, the part extending between the slit and the light receiver, is a first detection section, in which the detection light is blocked by the first substrate held by the holder, and part of the second light guiding section, the part extending through space between the one distal end portion and the other distal end portion, is a second detection section, in which the detection light is blocked by the second substrate detected by the mapping detector. an attachment plate; a hand including a proximal end portion fixed to one end of the attachment plate, a pair of distal end portions (10b, 10c) branching off from the proximal end portion, and a slit formed between the pair of distal end portions (10b, 10c) and the proximal end portion; a holder provided on the hand and configured to hold a first substrate; a mapping detector provided at the pair of distal end portions of the hand (10b, 10c), the mapping detector being configured to face a second substrate stored in a cassette and detect presence or absence of the second substrate; a light emitter, which is built-in in the attachment plate and which is configured to convert an electrical input to generate detection light; a light receiver, which is provided on an upper surface of the attachment plate and which is configured to receive the detection light and convert the detection light into an electrical output; and an optical path formed in the hand, through which the detection light emitted from the light emitter passes to be incident on the light receiver, wherein the optical path is formed such that the detection light is blocked by the first substrate held by the holder, and such that the detection light is blocked by the second substrate detected by the mapping detector, the optical path includes: a first light guiding section extending from the light emitter to a lower surface of one of the distal end portions of the hand; a second light guiding section extending from an end of the first light guiding section to a lower surface of the other distal end portion of the hand; and a third light guiding section extending from an end of the second light guiding section through the slit to the light receiver, a first light guiding section extending from the light emitter to a lower surface of one of the distal end portions of the hand; a second light guiding section extending from an end of the first light guiding section to a lower surface of the other distal end portion of the hand; and a third light guiding section extending from an end of the second light guiding section through the slit to the light receiver, part of the third light guiding section, the part extending between the slit and the light receiver, is a first detection section, in which the detection light is blocked by the first substrate held by the holder, and part of the second light guiding section, the part extending through space between the one distal end portion and the other distal end portion, is a second detection section, in which the detection light is blocked by the second substrate detected by the mapping detector.