A gas supply apparatus is provided and has first and second fittings and first and second tubes. The first fitting is provided to discharge a gas of a first pressure therethrough, while the second fitting is provided to discharge a gas of a second pressure therethrough. The first tube has one end to which a first connector connectable to the first fitting is attached and supplies the first-pressure gas to a first body cavity of a subject. The second tube has one end to which a second connector connectable to the second fitting is attached and supplies the second-pressure gas to a second body cavity of the subject. The apparatus may comprise an erroneous-connection preventing device preventing an erroneous connection including at least one of a connection of the first connector to the second fitting and a further connection of the second connector to the first fitting.