PURPOSE: To enable the detecting of sun set for a timepiece in a non-contact manner and automatically, by detecting changes in the quantity of light with a detection head section employing a plurality of projecting and light receiving fibers.

CONSTITUTION: A detection head section 1 is made up of ten piece each of projecting fibers 2 and light receiving fibers 3 and the fibers 2 are connected to a light source unit 4 to shine a date indicator through the fibers 2. The ten pieces of the fibers 3 are connected separately to a preamplifier substrate 5 and further to a drive controller 7 to rotate the data indicator via a detection circuit 6. Then, the data indicator of a timepiece is turned with the controller 7 and changes in the quantity of light are received with the fibers 3 about the brightness instantly the data is shifted. A signal corresponding to the changes in the quantity of light is amplified with a preamplifier substrate 5, after DC amplification thereof with the detection circuit 6, variation of the quantity is amplified with a differentiation circuit and only a signal exceeding a fixed level is outputted with a decision circuit. The output is read out with the controller 7 to stop the rotation of the data indicator.