A thin film forming method includes: a first operation of supplying a source gas at a first flow rate into a reactor; a second operation of purging the source gas in the reactor to an exhaust unit; a third operation of supplying a reactive gas at a second flow rate into the reactor; a fourth operation of supplying plasma into the reactor; and a fifth operation of purging the reactive gas in the reactor to the exhaust unit, wherein, during the second to fifth operations, the source gas is bypassed to the exhaust unit, and a flow rate of the source gas bypassed to the exhaust unit is less than the first flow rate. According to the thin film forming method, the consumption of the source gas and the reactive gas may be reduced, and the generation of reaction by-products in the exhaust unit may be minimized.