To provide a file structured so as to make it difficult to remove papers by a reader.

The retention mechanism 9 comprises: a securing piece 91 as a securing member which engages with a securing hole 6x of a card key 6 and retains the card key 6 as it is received in a permission element receiving part 7, when the card key 6 as an object for engagement, is received into the permission element receiving part 7 formed in the file body as a main body; a disengagement operating part 93 which is formed in the securing piece 91 and disengages the securing piece 91 from the securing hole 6x, when a manual operating force is applied to the part 93; and a spring 92 as an urging means for urging the card key 6, in its discharging direction, from the permission element receiving part 7, when the securing piece 91 is separated from the engagement hole 6x.