Each lock portion (15) has a front portion (23) including an anchoring pawl (24) and a back portion (25) extending in a direction opposite to the front portion. Therefore, when a slider (14) withdraws, each back portion comes into contact with the inside surface of a back wall (10) of a cassette shell (4) and each lock portion rotates in an unlocking direction. A bottom surface (109) defining each recess between engaging teeth (107) has a central portion which protrudes in a chevron shape. A corner of each protrusion has tapered surfaces (110') and (110''). An anchoring pawl of each lock portion has a flat surface that comes into contact with one inclined surface (109') of a chevron-shaped portion of any one of the recesses and one tapered surface (110') of a protrusion disposed behind the adjacent recess.