A sliding door apparatus comprising: a door (2a, 2b) that opens and closes a doorway by being moved horizontally; a first detecting apparatus that has: a light emitter (11, 12) that is disposed on a side portion of the doorway; and an imaging means (13, 14) that faces the light emitter (11, 12) from an opposite side of the doorway and captures images of the light emitter (11, 12), the first detecting apparatus detecting an object that is present in the doorway; characterized by further comprising: a second detecting apparatus that has: a light source (15, 17) that emits a light beam along a side portion of the doorway (1 a); and a photodetector (16, 18) that receives the light beam from the light source (15, 17), the second detecting apparatus detecting an object that is present in a vicinity of the side portion of the doorway; and a controlling apparatus (19, 21) that controls opening and closing of the door (2a, 2b) based on information from the first and second detecting apparatuses (22, 23), wherein the light emitter (11, 12) and the light source (15, 17) emit mutually-different wavelengths of light. a door (2a, 2b) that opens and closes a doorway by being moved horizontally; a first detecting apparatus that has: a light emitter (11, 12) that is disposed on a side portion of the doorway; and an imaging means (13, 14) that faces the light emitter (11, 12) from an opposite side of the doorway and captures images of the light emitter (11, 12), the first detecting apparatus detecting an object that is present in the doorway; a light emitter (11, 12) that is disposed on a side portion of the doorway; and an imaging means (13, 14) that faces the light emitter (11, 12) from an opposite side of the doorway and captures images of the light emitter (11, 12), the first detecting apparatus detecting an object that is present in the doorway; characterized by further comprising: a second detecting apparatus that has: a light source (15, 17) that emits a light beam along a side portion of the doorway (1 a); and a photodetector (16, 18) that receives the light beam from the light source (15, 17), the second detecting apparatus detecting an object that is present in a vicinity of the side portion of the doorway; and a controlling apparatus (19, 21) that controls opening and closing of the door (2a, 2b) based on information from the first and second detecting apparatuses (22, 23), wherein the light emitter (11, 12) and the light source (15, 17) emit mutually-different wavelengths of light. a second detecting apparatus that has: a light source (15, 17) that emits a light beam along a side portion of the doorway (1 a); and a photodetector (16, 18) that receives the light beam from the light source (15, 17), the second detecting apparatus detecting an object that is present in a vicinity of the side portion of the doorway; and a light source (15, 17) that emits a light beam along a side portion of the doorway (1 a); and a photodetector (16, 18) that receives the light beam from the light source (15, 17), the second detecting apparatus detecting an object that is present in a vicinity of the side portion of the doorway; and a controlling apparatus (19, 21) that controls opening and closing of the door (2a, 2b) based on information from the first and second detecting apparatuses (22, 23), wherein the light emitter (11, 12) and the light source (15, 17) emit mutually-different wavelengths of light.