A source gas supply unit includes a canister including a precursor accommodating space therein, and an inflow surface and an outflow surface which are open. The source gas supply unit includes a first lid configured to seal the inflow surface of the canister and having a gas inlet connected to the precursor accommodating space and a second lid configured to seal the outflow surface of the canister and having a gas outlet connected to the precursor accommodating space. The source gas supply unit includes a ring-shaped solid precursor located in the precursor accommodating space and including a gas flow path therein, which communicates with the gas inlet and the gas outlet. A cross-sectional area of the gas flow path increases from the inflow surface of the canister toward the outflow surface thereof.