A silicon nitride film forming method for forming a silicon nitride film on a substrate to be processed, includes forming a silicon nitride film doped with a predetermined amount of titanium by repeating, a predetermined number of times, forming a silicon nitride film by repeating, a first number of times, a process of causing a silicon source gas to be adsorbed onto the substrate and a process of nitriding the adsorbed silicon source gas with plasma of a nitriding gas, and forming a titanium nitride film by repeating, a second number of times, a process of causing a titanium source gas containing chlorine to be adsorbed onto the substrate and a process of nitriding the adsorbed titanium source gas with the plasma of the nitriding gas.