A sliding door apparatus comprising: a first door (21, 22) that opens and closes a first entrance by being slid horizontally; a second door (28, 29) that opens and closes a second entrance that faces the first entrance by being slid horizontally together with the first door (21, 22); imaging means (34, 35, 36, 73, 74) that is disposed beside a space between the first entrance and the second entrance, and that captures images across the space; a control apparatus that has an image processing and determining portion that determines presence or absence of an obstruction inside the space based on image data from the imaging means (34, 35, 36, 73, 74), and that controls opening and closing of the first and second doors (21, 22, 28, 29) depending on presence or absence of an obstruction; a light emitter (32, 71, 72) that has a vertically long light-emitting surface (32a, 49a, 50a, 51a, 52a, 71a, 72a) that is disposed so as to face the imaging means (34, 35, 36, 73, 74) across the space, wherein the imaging means (34, 35, 36, 73, 74) captures images of the light-emitting surface (32a, 49a, 50a, 51a, 52a, 71a, 72a), and at least one of the imaging means (34, 35, 36, 73, 74) or the light emitter (32, 71, 72) is mounted to the first door (21, 22) characterized in that the image processing and determining portion uses a reference length of an image of the light-emitting surface (32a, 49a, 50a, 51a, 52a, 71a, 72a) that corresponds to a position of the first door (21, 22), and determines that an obstruction is present if an image of the light-emitting surface (32a, 49a, 50a, 51a, 52a, 71a, 72a) is discontinuous, if a length of an image of the light-emitting surface (32a, 49a, 50a, 51a, 52a, 71a, 72a) becomes shorter, or if an image of the light-emitting surface (32a, 49a, 50a, 51a, 52a, 71a, 72a) disappears. a first door (21, 22) that opens and closes a first entrance by being slid horizontally; a second door (28, 29) that opens and closes a second entrance that faces the first entrance by being slid horizontally together with the first door (21, 22); imaging means (34, 35, 36, 73, 74) that is disposed beside a space between the first entrance and the second entrance, and that captures images across the space; a control apparatus that has an image processing and determining portion that determines presence or absence of an obstruction inside the space based on image data from the imaging means (34, 35, 36, 73, 74), and that controls opening and closing of the first and second doors (21, 22, 28, 29) depending on presence or absence of an obstruction; a light emitter (32, 71, 72) that has a vertically long light-emitting surface (32a, 49a, 50a, 51a, 52a, 71a, 72a) that is disposed so as to face the imaging means (34, 35, 36, 73, 74) across the space, wherein the imaging means (34, 35, 36, 73, 74) captures images of the light-emitting surface (32a, 49a, 50a, 51a, 52a, 71a, 72a), and at least one of the imaging means (34, 35, 36, 73, 74) or the light emitter (32, 71, 72) is mounted to the first door (21, 22) characterized in that the image processing and determining portion uses a reference length of an image of the light-emitting surface (32a, 49a, 50a, 51a, 52a, 71a, 72a) that corresponds to a position of the first door (21, 22), and determines that an obstruction is present if an image of the light-emitting surface (32a, 49a, 50a, 51a, 52a, 71a, 72a) is discontinuous, if a length of an image of the light-emitting surface (32a, 49a, 50a, 51a, 52a, 71a, 72a) becomes shorter, or if an image of the light-emitting surface (32a, 49a, 50a, 51a, 52a, 71a, 72a) disappears.