An illegal-act detecting mechanism detecting an illegal-act means attached to a paper sheet to be transported, comprising: an opening/closing member configured to permit passage of the paper sheet at an initial rotation position, and to block passage of the paper sheet at a non-initial rotation position deviated from the initial rotation position; a rotary member configured to coaxially and integrally rotate with the opening/closing member, wherein the rotary member includes at least one depressed portion on an outer peripheral edge; a driving member for driving the opening/closing member, which is arranged opposite to the rotary member, coaxially with the rotary member, and pivotally supported so as to be able to rotate relative to the rotary member; a drive transmission mechanism configured to transmit a driving force from the driving member to the rotary member; an illegal-act preventing motor configured to drive the driving member; a rotation-posture detecting unit configured to detect that the opening/closing member is at an initial rotation position; a control unit configured to control the illegal-act preventing motor, wherein the control unit is configured to turn off the illegal-act preventing motor when the rotation-posture detecting unit is detecting that the opening/closing member is at the initial rotation position; a rotation-posture detecting unit including a follow-up member configured by a rotatable roller that fits in a depressed portion and stops rotation of the rotary member mechanically when the rotary member is at the initial rotation position, and when the rotary member is moved from the initial rotation position to the non-initial rotation position, withdraws from the depressed portion and moves along an outer periphery of the rotary member; and an elastic member for elastically biasing the follow-up member in a direction in which the follow-up member comes in pressure contact with the outer peripheral edge of the rotary member, and a home-position detecting sensor that detects that the rotary member is at the initial rotation position only when the follow-up member completely fits in the depressed portion, wherein the drive transmission mechanism includes at least one driven piece (74,75,76) provided in the rotary member, at least one driving piece (92,93,96) that is provided in the driving member and configured to intermittently drive and rotate the rotary member by pressing the driven piece (74,75,76) directly or indirectly in a process of rotational transfer relative to the driven piece (74,75,76), and a buffer member configured to bias the driven piece (74,75,76) and the driving piece (92,93,96) in a direction away from each other. an opening/closing member configured to permit passage of the paper sheet at an initial rotation position, and to block passage of the paper sheet at a non-initial rotation position deviated from the initial rotation position; a rotary member configured to coaxially and integrally rotate with the opening/closing member, wherein the rotary member includes at least one depressed portion on an outer peripheral edge; a driving member for driving the opening/closing member, which is arranged opposite to the rotary member, coaxially with the rotary member, and pivotally supported so as to be able to rotate relative to the rotary member; a drive transmission mechanism configured to transmit a driving force from the driving member to the rotary member; an illegal-act preventing motor configured to drive the driving member; a rotation-posture detecting unit configured to detect that the opening/closing member is at an initial rotation position; a control unit configured to control the illegal-act preventing motor, wherein the control unit is configured to turn off the illegal-act preventing motor when the rotation-posture detecting unit is detecting that the opening/closing member is at the initial rotation position; a rotation-posture detecting unit including a follow-up member configured by a rotatable roller that fits in a depressed portion and stops rotation of the rotary member mechanically when the rotary member is at the initial rotation position, and when the rotary member is moved from the initial rotation position to the non-initial rotation position, withdraws from the depressed portion and moves along an outer periphery of the rotary member; and an elastic member for elastically biasing the follow-up member in a direction in which the follow-up member comes in pressure contact with the outer peripheral edge of the rotary member, and a home-position detecting sensor that detects that the rotary member is at the initial rotation position only when the follow-up member completely fits in the depressed portion, wherein the drive transmission mechanism includes at least one driven piece (74,75,76) provided in the rotary member, at least one driving piece (92,93,96) that is provided in the driving member and configured to intermittently drive and rotate the rotary member by pressing the driven piece (74,75,76) directly or indirectly in a process of rotational transfer relative to the driven piece (74,75,76), and a buffer member configured to bias the driven piece (74,75,76) and the driving piece (92,93,96) in a direction away from each other.