PURPOSE: To provide a method for accurately obtaining a skew of an irregularity of a tape fiber by measuring a light propagating time itself of the fiber in each fiber.

CONSTITUTION: A pulse light oscillated from a pulse LD light source 1 is incident to a 3dB coupler 3 to be demultiplexed, and incident at the other to one core of a sample tape fiber 6 through an optical connector 5. An incident pulse light is reflected at an open end of a fiber 8, and then incident to an optical sampling oscilloscope 2 through the coupler 3. On the other hand, the other demultiplexed light is reflected at an open end 9 of the coupler 3, and then similarly incident to the oscilloscope 2. Thus, a time interval of the reflected pulse lights multiplexed by the coupler 3 is measured thereby to measure a light propagating time in each fiber 8, thereby obtaining a skew of the fiber 6.