An analyzing apparatus comprising: an analyzing device having a microchannel structure for transferring a sample liquid to a measurement spot (17a, 17b, 17c) by a centrifugal force; a rotating unit for rotating the set analyzing device; a light source and a photodetector opposed to each other with the set analyzing device disposed between the light source and the photodetector; and a reading device configured to detect output light having passed through the measurement spot (17a, 17b, 17c) of the analyzing device, by using the photodetector, the analyzing device including at least one position detection mark having one of a reflective surface, a refractive surface, and a light shielding surface that is configured to block the output light incident on the photodetector from the light source when the output light reaches a rotation detection position just before or behind the measurement spot (17a, 17b, 17c), wherein the reading device includes a signal processor configured to extract the output light just having passed through the measurement spot (17a, 17b, 17c) of the analyzing device and calculate a component of the sample liquid based on a detection signal of the photodetector, characterized in that the analyzing device includes a starting point position mark indicating a position of a starting point in a direction of rotation, the analyzing apparatus includes a starting point sensor configured to detect the starting point position mark and output a starting point signal, and the signal processor is configured to turn on or off the light source based on the starting point signal and a rotation detection signal obtained from the rotating unit. an analyzing device having a microchannel structure for transferring a sample liquid to a measurement spot (17a, 17b, 17c) by a centrifugal force; a rotating unit for rotating the set analyzing device; a light source and a photodetector opposed to each other with the set analyzing device disposed between the light source and the photodetector; and a reading device configured to detect output light having passed through the measurement spot (17a, 17b, 17c) of the analyzing device, by using the photodetector, the analyzing device including at least one position detection mark having one of a reflective surface, a refractive surface, and a light shielding surface that is configured to block the output light incident on the photodetector from the light source when the output light reaches a rotation detection position just before or behind the measurement spot (17a, 17b, 17c), wherein the reading device includes a signal processor configured to extract the output light just having passed through the measurement spot (17a, 17b, 17c) of the analyzing device and calculate a component of the sample liquid based on a detection signal of the photodetector, characterized in that the analyzing device includes a starting point position mark indicating a position of a starting point in a direction of rotation, the analyzing apparatus includes a starting point sensor configured to detect the starting point position mark and output a starting point signal, and the signal processor is configured to turn on or off the light source based on the starting point signal and a rotation detection signal obtained from the rotating unit.