A multi-hardness cushion having a ventilation structure, comprising: a support cushion provided in a form of a vehicle seat and having an insert recess formed in a portion where a passenger's buttocks are placed when he or she is seated, with stepped portions being formed on both sides of the insert recess; a friction cushion detachably provided in the insert recess of the support cushion so that the passenger's buttocks are placed thereon when he or she is seated, and having a locking part that is locked to each of the stepped portions, characterized by a first ventilation unit having a first air flow channel provided in the insert recess, a plurality of first side flow channels (137a, 137b) provided in each of the stepped portions, and an air inlet opening provided in the first air flow channel; and a second ventilation unit having a second air flow channel that is formed in the friction cushion to face the first air flow channel, a plurality of second air flow holes that are formed along the second air flow channel to penetrate the friction cushion, and a plurality of second side flow holes formed in each of the locking parts to communicate with the plurality of first side flow channels (137a, 137b), wherein air introduced into the air inlet opening passes through a central portion of the friction cushion via the plurality of second air flow holes while flowing through the first and second air flow channels (133, 143), and ventilates the seat while passing through both sides of the friction cushion via the plurality of first side flow channels (137a, 137b) and the plurality of second side flow holes (147a, 147b). a support cushion provided in a form of a vehicle seat and having an insert recess formed in a portion where a passenger's buttocks are placed when he or she is seated, with stepped portions being formed on both sides of the insert recess; a friction cushion detachably provided in the insert recess of the support cushion so that the passenger's buttocks are placed thereon when he or she is seated, and having a locking part that is locked to each of the stepped portions, characterized by a first ventilation unit having a first air flow channel provided in the insert recess, a plurality of first side flow channels (137a, 137b) provided in each of the stepped portions, and an air inlet opening provided in the first air flow channel; and a second ventilation unit having a second air flow channel that is formed in the friction cushion to face the first air flow channel, a plurality of second air flow holes that are formed along the second air flow channel to penetrate the friction cushion, and a plurality of second side flow holes formed in each of the locking parts to communicate with the plurality of first side flow channels (137a, 137b), wherein air introduced into the air inlet opening passes through a central portion of the friction cushion via the plurality of second air flow holes while flowing through the first and second air flow channels (133, 143), and ventilates the seat while passing through both sides of the friction cushion via the plurality of first side flow channels (137a, 137b) and the plurality of second side flow holes (147a, 147b).