Machine for producing bags of flexible material comprising: - a reel-holder group which supports in a removable manner at least one reel (14, 16) capable of supplying at least one film of flexible material; - a welding group provided with at least one upper welding plate and with at least one corresponding lower welding plate which are heated and provided with corresponding linear welding tracks, said upper and lower welding plates acting on said film according to a vertical alternate movement, pinching said film along one or more linear portions; - at least one cutting group (24, 26) which separates from each other the single bags just obtained further to the welding operation on said film, wherein the lower welding plate is operatively connected to a base, whereas the upper welding plate is mobile with an alternate motion along a plurality of vertical guiding columns and is constrained at the top to four moving means (36, 38) arranged on the vertices of a rectangle and associated with a cover fixed above said vertical guiding columns, each of the single moving means (36, 38) being made of a screw which vertically slides inside a respective female screw obtained across the cover and is driven by a respective motor integral with the cover, each screw being provided with control means (48, 50) of its current position, wherein the lower welding plate is provided with at least three sensor means made of load cells capable of instantly detecting the compression load, measured at each point of force, which the upper welding plate imparts onto said lower welding plate, wherein each motor is operatively connected to an electronic control unit capable of managing the functioning of all motors, said electronic control unit being capable of activating simultaneously and in perfect synchrony all motors, as well as of maintaining all functioning parameters controlled in order for the movement of the upper welding plate to be always regular and uniform, and said sensor means being operatively connected to the electronic control unit in order to send to said electronic control unit the signals relating to the compression load, measured at all points of force, derived from each "stroke" performed by said upper welding plate onto said lower welding plate, said signals being interpreted by said electronic control unit in order to manage independently the functioning of each single motor, and said control means (48, 50) being operatively connected to the electronic control unit in order to verify that said current position coincides with the controlled position and with the current position of all remaining screws and to verify the perfect synchronization of the simultaneous movement of the screws, consequently guaranteeing the homogeneity of movement of the upper welding plate. - a reel-holder group which supports in a removable manner at least one reel (14, 16) capable of supplying at least one film of flexible material; - a welding group provided with at least one upper welding plate and with at least one corresponding lower welding plate which are heated and provided with corresponding linear welding tracks, said upper and lower welding plates acting on said film according to a vertical alternate movement, pinching said film along one or more linear portions; - at least one cutting group (24, 26) which separates from each other the single bags just obtained further to the welding operation on said film,