A passenger boarding bridge comprising: a rotunda configured to be connected to a terminal building and supported in a horizontally rotatable manner; a tunnel unit whose proximal end is connected to the rotunda, the tunnel unit being extendable and retractable, the tunnel unit being rotatable about a rotational vertical axis of the rotunda; a travel device that supports the tunnel unit and includes travel wheels configured to travel forward and backward, the travel device being configured such that a travel direction of forward travel of the travel wheels and a travel direction of backward travel of the travel wheels are changeable; a cab provided at a distal end of the tunnel unit, the cab being configured to be docked with an aircraft; and a determiner configured to, at a time before undocking the cab docked with the aircraft from the aircraft, perform determination whether or not the travel direction of the backward travel of the travel wheels at the time is a direction within a smooth undocking range (θss), wherein: the smooth undocking range is an angle range that is, in a plan view, not less than 90 degrees as calculated clockwise with respect to a center line (Ed) of the tunnel unit before undocking the cab from the aircraft, and is, in the plan view, set based on predetermined information within a range less than an angle that is obtained by adding 180 degrees to one of the following angles: an angle that is formed by a tangent line and that is calculated clockwise with respect to the center line (Ed) of the tunnel unit before undocking the cab from the aircraft, the tangent line extending horizontally and touching a cab-docked part of the aircraft; an angle that is formed by a straight line and that is calculated clockwise with respect to the center line (Ed) of the tunnel unit before undocking the cab from the aircraft, the straight line extending along a distal end edge of the cab; an angle (Wx) that is formed by a fuselage guide line drawn on an apron and that is calculated clockwise with respect to the center line (Ed) of the tunnel unit before undocking the cab from the aircraft; and an angle that is formed by an aircraft axis line of the aircraft and that is calculated clockwise with respect to the center line of the tunnel unit before undocking the cab from the aircraft. a rotunda configured to be connected to a terminal building and supported in a horizontally rotatable manner; a tunnel unit whose proximal end is connected to the rotunda, the tunnel unit being extendable and retractable, the tunnel unit being rotatable about a rotational vertical axis of the rotunda; a travel device that supports the tunnel unit and includes travel wheels configured to travel forward and backward, the travel device being configured such that a travel direction of forward travel of the travel wheels and a travel direction of backward travel of the travel wheels are changeable; a cab provided at a distal end of the tunnel unit, the cab being configured to be docked with an aircraft; and a determiner configured to, at a time before undocking the cab docked with the aircraft from the aircraft, perform determination whether or not the travel direction of the backward travel of the travel wheels at the time is a direction within a smooth undocking range (θss), wherein: the smooth undocking range is an angle range that is, in a plan view, not less than 90 degrees as calculated clockwise with respect to a center line (Ed) of the tunnel unit before undocking the cab from the aircraft, and is, in the plan view, set based on predetermined information within a range less than an angle that is obtained by adding 180 degrees to one of the following angles: an angle that is formed by a tangent line and that is calculated clockwise with respect to the center line (Ed) of the tunnel unit before undocking the cab from the aircraft, the tangent line extending horizontally and touching a cab-docked part of the aircraft; an angle that is formed by a straight line and that is calculated clockwise with respect to the center line (Ed) of the tunnel unit before undocking the cab from the aircraft, the straight line extending along a distal end edge of the cab; an angle (Wx) that is formed by a fuselage guide line drawn on an apron and that is calculated clockwise with respect to the center line (Ed) of the tunnel unit before undocking the cab from the aircraft; and an angle that is formed by an aircraft axis line of the aircraft and that is calculated clockwise with respect to the center line of the tunnel unit before undocking the cab from the aircraft. an angle that is formed by a tangent line and that is calculated clockwise with respect to the center line (Ed) of the tunnel unit before undocking the cab from the aircraft, the tangent line extending horizontally and touching a cab-docked part of the aircraft; an angle that is formed by a straight line and that is calculated clockwise with respect to the center line (Ed) of the tunnel unit before undocking the cab from the aircraft, the straight line extending along a distal end edge of the cab; an angle (Wx) that is formed by a fuselage guide line drawn on an apron and that is calculated clockwise with respect to the center line (Ed) of the tunnel unit before undocking the cab from the aircraft; and an angle that is formed by an aircraft axis line of the aircraft and that is calculated clockwise with respect to the center line of the tunnel unit before undocking the cab from the aircraft.