To provide an engine start permission mechanism with a key cylinder enabling its internal structure to be shared with a smart ignition device.

A rotor 42 is rotatably housed in a rotor case 41 which constitutes the key cylinder 11. A plurality of tumblers 45 are disposed along the axial direction of the rotor 42. The upper end edges 45a of the tumblers 45 are aligned by a portable machine 61. The upper end edges 45a are aligned also by an alignment means 71 which is actuated on condition that ID codes match. As a result, the rotor 42 is rendered rotatable.