An electric bicycle comprising: a human power driving ring body that rotates with a crank and outputs a human driving force including a pedal force; a driving force transmitter rotatably wound around the human power driving ring body and the rear ring body in an endless manner; an electric motor acting as a driving source for generating an auxiliary driving force; an auxiliary driving ring body that receives the auxiliary driving force generated in the electric motor and transmits the auxiliary driving force to the driving force transmitter while being engaged with the driving force transmitter; and a tensioner device that increases a winding angle of the driving force transmitter with respect to the auxiliary driving ring body and applies a tension to the driving force transmitter so as to absorb slack on a lower portion of the driving force transmitter, and which has a tension ring body engaged with the driving force transmitter; a support member pivotally supporting the tension ring body; an urging member that applies an urging force to the tension ring body so as to move the tension ring body in a direction of increasing the tension of the driving force transmitter characterized by a rear ring body and a coaster brake provided on a rear wheel, and further characterized in that the tensioner device comprises: a positioning mechanism that adjustably regulates a limit position of movement of the tension ring body at multiple positions including an intermediate position in a pivoting range of the support member, when the coaster brake is started by rotating pedals in a direction opposite to a forward rotation direction, allowing the driving force transmitter to receive a tension for moving the tension ring body in a direction opposite to an urging direction of the urging force. a human power driving ring body that rotates with a crank and outputs a human driving force including a pedal force; a driving force transmitter rotatably wound around the human power driving ring body and the rear ring body in an endless manner; an electric motor acting as a driving source for generating an auxiliary driving force; an auxiliary driving ring body that receives the auxiliary driving force generated in the electric motor and transmits the auxiliary driving force to the driving force transmitter while being engaged with the driving force transmitter; and a tensioner device that increases a winding angle of the driving force transmitter with respect to the auxiliary driving ring body and applies a tension to the driving force transmitter so as to absorb slack on a lower portion of the driving force transmitter, and which has a tension ring body engaged with the driving force transmitter; a support member pivotally supporting the tension ring body; an urging member that applies an urging force to the tension ring body so as to move the tension ring body in a direction of increasing the tension of the driving force transmitter characterized by a rear ring body and a coaster brake provided on a rear wheel, and further characterized in that the tensioner device comprises: a positioning mechanism that adjustably regulates a limit position of movement of the tension ring body at multiple positions including an intermediate position in a pivoting range of the support member, when the coaster brake is started by rotating pedals in a direction opposite to a forward rotation direction, allowing the driving force transmitter to receive a tension for moving the tension ring body in a direction opposite to an urging direction of the urging force. a positioning mechanism that adjustably regulates a limit position of movement of the tension ring body at multiple positions including an intermediate position in a pivoting range of the support member, when the coaster brake is started by rotating pedals in a direction opposite to a forward rotation direction, allowing the driving force transmitter to receive a tension for moving the tension ring body in a direction opposite to an urging direction of the urging force.