The durability of an automatic transmission is greatly affected by its lubrication properties. However, it is difficult to estimate exactly the flow rate of lubricating oil at each part of the automatic transmission, because of its structural complexity under rotating conditions. This paper presents a simple model which can estimate the flow rate of lubricating oil in the automatic transmission. Using this method, we can obtain various information about the lubricating state at every part of the automatic transmission and can help our judgement of whether lubrication is sufficient. To test this model and its results, we tried some experiments and examined the lubricating state by measuring discharge rate of oil. The estimated data is in good agreement with the experimental data. This study should assist in the optimal design of the lubricating system in the automatic transmission.