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.