A roof airbag apparatus may include: a first-row deployment part deployed from the front to the rear of a roof so as to cover a front area, when gas is supplied from a gas supply part; a second-row deployment part coupled to a front end of the first-row deployment part so as to communicate with the first-row deployment part, and configured to cover a rear area which is deployed to the rear when the first-row deployment part is deployed; and an auxiliary deployment part coupled to a top or bottom surface of the first-row deployment part so as to communicate with the first-row deployment part, and configured to increase/decrease the top-to-bottom thickness of the first-row deployment part, when deployed.