We have investigated the defects observed in the multimode waveguide fabricated on the flexible copper clad laminate using vacuum lamination of film-type raw material, UV exposure and development processes. The excess loss caused by the defects was estimated using the simulation method and the estimated values were found to be well matched with the measured ones except the trapped void type defect which gives excess loss larger than 1 dB by a single defect. From these results, we proposed how the allowable defect size can be determined using the visual inspection system.