A packing machine for producing a pack of smoking articles with a cylindrical shape and comprising: a horizontal feeding channel and a forming unit for forming groups of smoking articles provided with a hopper adapted to contain a compact mass of cigarettes; the hopper has a vertical longitudinal axis and comprises: at least one group of grooves within which the smoking articles are arranged in stacks and end in an output mouth, from which a group of smoking articles can be extracted; a free chamber which is arranged above the grooves and from which the smoking articles enter directly into the grooves; an upper input to which the smoking articles are fed by the horizontal feeding channel; and two channels, which are distinct and placed in a mirrorlike manner relative to the vertical longitudinal axis, are arranged on the right and on the left, respectively, of the axis, and connect the upper input to the free chamber; wherein each channel comprises an initial portion and a final portion; wherein the two initial portions of the channels diverge between one another from the top to the bottom and the two final portions converge between one another from the top to the bottom; and wherein each channel is arranged higher than the output mouth in the direction of the longitudinal axis, has a pair of opposite side walls against which, cylindrical side walls of the smoking articles rest, and along which the smoking articles descend, due to gravity, towards the output mouth until reaching the free chamber directly; the packing machine is characterized in that the opposite side walls of the initial portion of each channel is defined by a pair of opposite first belt conveyors and the opposite side walls of the final portion of each channel is defined by a pair of opposite second belt conveyors. at least one group of grooves within which the smoking articles are arranged in stacks and end in an output mouth, from which a group of smoking articles can be extracted; a free chamber which is arranged above the grooves and from which the smoking articles enter directly into the grooves; an upper input to which the smoking articles are fed by the horizontal feeding channel; and two channels, which are distinct and placed in a mirrorlike manner relative to the vertical longitudinal axis, are arranged on the right and on the left, respectively, of the axis, and connect the upper input to the free chamber; wherein each channel comprises an initial portion and a final portion; wherein the two initial portions of the channels diverge between one another from the top to the bottom and the two final portions converge between one another from the top to the bottom; and wherein each channel is arranged higher than the output mouth in the direction of the longitudinal axis, has a pair of opposite side walls against which, cylindrical side walls of the smoking articles rest, and along which the smoking articles descend, due to gravity, towards the output mouth until reaching the free chamber directly; the packing machine is characterized in that the opposite side walls of the initial portion of each channel is defined by a pair of opposite first belt conveyors and the opposite side walls of the final portion of each channel is defined by a pair of opposite second belt conveyors.