A substrate conveying system for an electronic component mounting device, comprising: a mounting device conveyor configured to convey a printed substrate from one end portion to the other end portion of the electronic component mounting device in a first direction that is horizontal; an upstream-side conveyor configured to unload the printed substrate from an upstream-side apparatus positioned upstream of the electronic component mounting device in a conveying direction of the printed substrate; a downstream-side conveyor configured to load the printed substrate onto a downstream-side apparatus positioned downstream of the electronic component mounting device in the conveying direction of the printed substrate; and a buffer conveyor (11, 12) arranged between at least one of the upstream-side conveyor and the downstream-side conveyor and the mounting device conveyor, wherein the mounting device conveyor has a function of moving the printed substrate in a second direction that is horizontal and perpendicular to the conveying direction, the upstream-side conveyor is spaced apart from the downstream-side conveyor in the second direction, the buffer conveyor (11, 12) has a function of conveying the printed substrate in the first direction, and a function of moving the printed substrate in the second direction, the printed substrate is transferred from one of the mounting device conveyor and the buffer conveyor (11, 12) to the other in a state in which the mounting device conveyor and the buffer conveyor (11, 12) are positioned in the same position in the second direction, and the buffer conveyor (11, 12) includes a downstream-side buffer conveyor installed between the downstream-side conveyor and the mounting device conveyor, wherein the substrate conveying system further comprises a controller that controls operations of the mounting device conveyor and the buffer conveyor (11, 12) characterized in that, the controller configured to: i) determine whether the downstream-side buffer conveyor is waiting for unloading of the printed substrate to the downstream side conveyor, ii) determine whether a processed printed substrate and a printed substrate to be processed next exist on the mounting device conveyor, when the controller has determined that the downstream-side buffer conveyor is waiting for unloading of the printed substrate, iii) move the mounting device conveyor to a next position in the second direction where a step next to the printed substrate unloading step is performed, when the controller has determined that the processed printed substrate and the printed substrate to be processed next exist on the mounting device conveyor, and iv) move the downstream-side buffer conveyor having unloaded the printed substrate to the same position in the second direction as that of the mounting device conveyor. a mounting device conveyor configured to convey a printed substrate from one end portion to the other end portion of the electronic component mounting device in a first direction that is horizontal; an upstream-side conveyor configured to unload the printed substrate from an upstream-side apparatus positioned upstream of the electronic component mounting device in a conveying direction of the printed substrate; a downstream-side conveyor configured to load the printed substrate onto a downstream-side apparatus positioned downstream of the electronic component mounting device in the conveying direction of the printed substrate; and a buffer conveyor (11, 12) arranged between at least one of the upstream-side conveyor and the downstream-side conveyor and the mounting device conveyor, i) determine whether the downstream-side buffer conveyor is waiting for unloading of the printed substrate to the downstream side conveyor, ii) determine whether a processed printed substrate and a printed substrate to be processed next exist on the mounting device conveyor, when the controller has determined that the downstream-side buffer conveyor is waiting for unloading of the printed substrate, iii) move the mounting device conveyor to a next position in the second direction where a step next to the printed substrate unloading step is performed, when the controller has determined that the processed printed substrate and the printed substrate to be processed next exist on the mounting device conveyor, and iv) move the downstream-side buffer conveyor having unloaded the printed substrate to the same position in the second direction as that of the mounting device conveyor.