PURPOSE: To monitor an optical signal and attain the advantage in designing by arranging condenser lenses, which are provided on one side of two optical fibers, apart from each other so that they face each other and arranging two pairs of these condenser lenses crosswise in the gap part and arranging a spectroscope freely attachably and detachably to switch the quantity of light and the optical path.

CONSTITUTION: 90% optical signal energy transmitted through a loop-shaped optical fiber 1A is reflected on a half mirror 7 and is transmitted to a loop-shaped optical fiber 1B and is transmitted to a following terminal equipment. Meanwhile, 10% remainder optical signal energy is transmitted through the half mirror 7 and reaches an input device 9-1 of a terminal equipment 9. When the terminal equipment enters into the normal operation, the input device detects reference signal from the optical signal and grasps the present timing of the system and connects the terminal equipment to the loop-shaped optical fiber and removes the half mirror 7 from the optical path during this time. In this state, all of the optical signal transmitted through the loop- shaped optical fiber 1A is inputted to the terminal equipment 9 through optical fibers 2-1 and 2-3 and is transmitted to the loop-shaped optical fiber 1B through optical fibers 2-4 and 2-2.