A cutter mechanism has a main unit supporting a movable blade and a motor for driving the movable blade. A detachable unit is attachable and detachable from the main unit and supports a fixed blade for cutting a sheet while sandwiching the sheet together with the movable blade. A holder holds the fixed blade with respect to the movable blade in an inclined state so that a cutting edge of the fixed blade forms a predetermined cutting angle with respect to a cutting edge of the movable blade. A holder support member supports the fixed blade movably in a direction in which the movable blade cutting edge moves close to and away from the fixed blade cutting edge. A biasing member biases the holder in a direction so that the fixed blade cutting edge is brought into press-contact with the movable blade cutting edge.