The method of manufacturing a gas barrier film feeds long lengths of a substrate and forms a silicon nitride film as the gas barrier film on the substrate by a capacitively coupled plasma-enhanced CVD technique while transporting the substrate in a longitudinal direction. Gaseous raw materials using in the forming step of the silicon nitride film includes at least silane gas and ammonia gas, and a ratio P/Q [W/sccm] is not less than 1 when a flow rate of the silane gas is denoted as Q [sccm] and a power input for generating a capacitively coupled plasma is denoted as P [W], a tension applied to the substrate transported between two transporting elements is not more than 100 [N/m], and a pair of electrodes for at least forming the silicon nitride film on the substrate is interposed between the two transporting elements.