A horizontal-type automatic packaging machine comprising: a conveyor moving a plurality of gripper-holding carriages in an advancing direction along a closed path having a working section and a return section; a package supplying unit supplying empty flexible packages to said gripper-holding carriages at the beginning of said working section at least one filling station and at least one sealing station filling and sealing the flexible packages, respectively, in the working section; and an output unit receiving the filled and sealed flexible packages delivered by the gripper-holding carriages at the end of the working section; wherein each of the gripper-holding carriages comprises: a base structure connected to the conveyor; a pair of left and right gripper arms (8a, 8b) extending from said base structure perpendicular to said advancing direction, said left and right gripper arms (8a, 8b) having respective left and right proximal ends (17a, 17b) slidingly coupled to at least one proximal horizontal guide parallel to the advancing direction fixed to the base structure and respective left and right distal ends (18a, 18b); at least two pairs of left and right grippers (47a, 47b) mounted in the left and right gripper arms (8a, 8b), said left and right grippers (47a, 47b) being operable for holding at least two flexible packages arranged in parallel; and a driving mechanism connected to said left and right proximal ends (17a, 17b) of the left and right gripper arms (8a, 8b) and moving the left and right gripper arms (8a, 8b) in opposite directions in relation to the base structure, moving one closer to the other for opening the flexible packages and moving one away from the other for closing the flexible packages, characterized in that: the base structure comprises an extension frame supporting at least one distal horizontal guide parallel to the advancing direction; said left and right distal ends (18a, 18b) of the left and right gripper arms (8a, 8b) are slidingly coupled to said distal horizontal guide; and said driving mechanism comprises a mechanical movement transmission transmitting the movement of the left and right proximal ends (17a, 17b) of the left and right gripper arms (8a, 8b) to the respective left and right distal ends (18a, 18b) of the left and right gripper arms (8a, 8b). a conveyor moving a plurality of gripper-holding carriages in an advancing direction along a closed path having a working section and a return section; a package supplying unit supplying empty flexible packages to said gripper-holding carriages at the beginning of said working section at least one filling station and at least one sealing station filling and sealing the flexible packages, respectively, in the working section; and an output unit receiving the filled and sealed flexible packages delivered by the gripper-holding carriages at the end of the working section; a base structure connected to the conveyor; a pair of left and right gripper arms (8a, 8b) extending from said base structure perpendicular to said advancing direction, said left and right gripper arms (8a, 8b) having respective left and right proximal ends (17a, 17b) slidingly coupled to at least one proximal horizontal guide parallel to the advancing direction fixed to the base structure and respective left and right distal ends (18a, 18b); at least two pairs of left and right grippers (47a, 47b) mounted in the left and right gripper arms (8a, 8b), said left and right grippers (47a, 47b) being operable for holding at least two flexible packages arranged in parallel; and a driving mechanism connected to said left and right proximal ends (17a, 17b) of the left and right gripper arms (8a, 8b) and moving the left and right gripper arms (8a, 8b) in opposite directions in relation to the base structure, moving one closer to the other for opening the flexible packages and moving one away from the other for closing the flexible packages, the base structure comprises an extension frame supporting at least one distal horizontal guide parallel to the advancing direction; said left and right distal ends (18a, 18b) of the left and right gripper arms (8a, 8b) are slidingly coupled to said distal horizontal guide; and said driving mechanism comprises a mechanical movement transmission transmitting the movement of the left and right proximal ends (17a, 17b) of the left and right gripper arms (8a, 8b) to the respective left and right distal ends (18a, 18b) of the left and right gripper arms (8a, 8b).