This study describes about the optical monitoring system for laser welding to detect in process welding point, condition, and welding mechanism by using simple apparatus. Two photo diodes and additional optical devices were attached to the end effector of welding robot which holds laser torch. A series of monitoring experiments at several welding positions have been performed and the results showed nonlinear relationship between distance from groove and differential ratio of two photo diodes. A newly model for laser welding was proposed to describe experimental results. This model assumes that emitting light has some energy distribution, and it is changed with detection ratio decided by surface condition. The results of simulation suggested that proposed model has validity to explain this mechanism.