Bicycle single-acted gear-shifting device (12), comprising one front or rear derailleur (10, 20) coupling a ratchet indexing mechanism (13) acted by only one single-acting slave-connector (110, 120) attached to said ratchet indexing mechanism (13), operated by a unidirectional operator mechanism (15) with optional multiple unidirectional handle operator units (30). Said ratchet indexing mechanism (13) removably defines and holds a plurality of predetermined working stable positions of the derailleur's cage-plate (102,202) for each gearshift, and it comprises a ratchet-gear (1) with a pawls' system. And said unidirectional operator mechanism (15) comprises optional multiple unidirectional operator units (30) connected in parallel with the only single-acting slave-connector (110, 120), in order to indistinctly operate it, by means of a branched single-acting drive circuit (115, 123) which can be arranged either using a hydraulic circuit (115, 115′, 115″) acting by pushing, or using flexible cables (123,123′) and housings (124) to act by pulling.