PURPOSE: To easily and surely install ribbon mask by a method wherein part for controlling the movement to the other side in the width direction of ribbon of ribbon mask is provided under the state being projected in the thickness of the ribbon mask beyond the width of a slit groove.

CONSTITUTION: The insertingly engaging piece 8 side of ribbon mask 6 is faced between stepped parts 24 and 25 and the end part of a tonguelike piece 18 to the inlet of groove 11 and then moved to one side in the width direction of ribbon in the direction along the plane of the ribbon so as to insertingly engage the insertingly engaging piece 8 inserted in the groove 11 in an insertingly engaging hole 19 under the state that the ribbon mask 6 is elastically curvedly deformed. After that, when the notched end edge of the ribbon mask 6 exceeds the stepped parts 24 and 25, the curved deformation of the ribbon mask 6 is released, resulting in inserting the notched part of the ribbon mask 6 and an engaging piece 26 from the direction of the thickness of the ribbon mask to the stepped parts 24 and 25 side. After that, the posture of the ribbon mask 6 is controlled by the tip parts of guide plates, resulting in installing the ribbon mask on the cassette case side in the predetermined posture, which curves outwards in the direction of the thickness of the ribbon mask.