Provided is a transmission gear box for bikes including: crank arms that receive a rotational driving force from rotatable pedals of each bike and transmits the received rotational driving force to a main shaft; a transmission gear unit that receives the rotational driving force from the main shaft and performs a transmission mode shifting operation at a gear tooth ratio according to a selected transmission mode gearshift; a lever manipulation unit that selects one of the transmission mode gearshifts of the transmission gear unit according to manipulation of a transmission mode shift lever, to thus manipulate one of cables connected to the respective transmission mode gearshifts; and a clutching unit that is operated by one cable selected by the lever manipulation unit to thus make one of ring gears of the transmission gear unit fixed in a gear tooth engagement coupling manner and to thereby manipulate a transmission mode shifting process of the transmission gear unit.