To provide a method for designing a collimator array device which can reduce an insertion loss due to the change of optical path length.

When a beam waist is positioned in the intermediate position (d1=L/2) between the flat plate micro lens of an emitting side and the flat plate micro lens of a light receiving side, a bilaterally symmetrical optical path is obtained. Thus, the interval (d0) between the fiber array of the emitting side and the flat plate micro lens of the emitting side can be made into the interval between a fiber array of the light receiving side and a flat plate micro lens of the light receiving side as they are, thus the design of the collimator array device is simplified. Then, when the interval (d0) acting as d1=L/2 is calculated, two values, (d0-2) and (d0-4), constitutes the interval acting as d1=L/2. Then, the insertion loss due to a deviation during coupling can be reduced by selecting a shorter value (d0-2).