The invention provides a free bidirectional clutch utilizing gear idle. By the use of the gear capable of idling, rotation of an input shaft can be transmitted to an output shaft and rotation transmission from the output shaft to the input shaft can be cut off. An input shaft and an output shaft with the same rotation axis as the center are disposed in a housing incapable of rotating; a concentric gear is formed on the output shaft; the periphery of the concentric gear is equipped with a planetary gear body of an external gear engaged with the concentric gear; a planetary gear body shaft is supported on a planetary rack; the planetary gear is provided with a protruding part protruding along the axis; the protruding part is configured in a position, where the protruding part can be buckled with an input rotation body formed on the input shaft; a linear part is formed in cross sections of the protruding part and the input rotation body; upon rotation of the input shaft, revolution of the planetary gear is stopped and the planetary gear is locked and rotation can be transmitted to the output shaft; upon rotation of the output shaft, the planetary gear idles and rotation to the input shaft can be cut off.