A roof mounted airbag is proposed. The roof mounted airbag includes: a fixed chamber having an upper portion fixed to a roof of a vehicle and configured to be expanded downward when gas is supplied into the fixed chamber; a first separated chamber connected to the fixed chamber, having an inside that communicates with an inside of the fixed chamber to be supplied with the gas from the fixed chamber, and configured to be expanded downward when the gas is supplied into the first separated chamber; and a second separated chamber connected to the fixed chamber while being spaced apart from the first separated chamber, having an inside that communicates with the inside of the fixed chamber to be supplied with the gas from the fixed chamber, and configured to be expanded downward when the gas is supplied into second separated chamber.