This article deals with the expressions of friction force of a four-point contact linear ball bearing with two rows under low-speed operation. First, the slip velocity on a contact area and the equilibrium of moments acting on a ball were theoretically examined. Then, an expression for the location of the pure rolling point (where the slip velocity is zero) was derived. Using the location of the pure rolling point, expressions of the friction force were developed. Next, to confirm the validity of the developed expressions, the friction force of four-point contact linear ball bearings with two rows was measured. The results showed that the developed expressions for the friction force in this study are effective. The developed expression would be applicable for estimating the friction force as a basic performance in the design procedure and for checking the product quality in bearing manufacturing.