This paper presents a method to analyze the production efficiency of automatic assembling lines with branches. The method is based upon an analysis of the fundaments of assembling lines which are composed of two assembling stations with an intermediate buffer, as mentioned in the previouse reports. An assembling station (ST.M+1) is branched to M-stations (ST.i;i=1, 2…, M) with each intermediate buffer. An equation for the interrupted time is introduced by considerations for the interferences with ST.M + I and ST.i. The equation of those stoppage times for ST.M+1 and ST.i organizes simultaneous equations for stowage ratios of intermediate buffers. The production efficiency of branched lines is given by solutions of those equations. Some examples are calculated numerically and simulations are performed by a degital computer for those. From the results, it is shown that the method is valuable for estimating the production efficiency of automatic assembling lines with branches.