A driving unit for use in an electric assist bicycle, comprising: a crankshaft; a motor; a transmission shaft extending parallel to a central axis of the crankshaft; and the motor including: a rotor; an output shaft configured to rotate together with the rotor and extending parallel to the central axis of the crankshaft; and an output gear located on the output shaft, the driving unit further comprising: a first transmission gear located on the transmission shaft and configured to engage the output gear, the first transmission gear including more cogs than the output gear does; a second transmission gear located on the transmission shaft; and a driven gear located on the crankshaft and configured to engage the second transmission gear, the driven gear including more cogs than the second transmission gear does, wherein, if out of the motor, first transmission gear and driven gear, a first component is a component with the largest diameter as viewed in an axial direction of the crankshaft, a second component is a component with the second largest diameter, and a third component is a component with the smallest diameter, characterized by a substrate as a printed circuit board, including a mounting surface expanding in directions perpendicular to the central axis of the crankshaft, the substrate is located between the first and second components as measured in an axial direction of the crankshaft, wherein the substrate is located in the same position as the third component as measured in an axial direction of the crankshaft. a crankshaft; a motor; a transmission shaft extending parallel to a central axis of the crankshaft; and the motor including: a rotor; an output shaft configured to rotate together with the rotor and extending parallel to the central axis of the crankshaft; and an output gear located on the output shaft, a rotor; an output shaft configured to rotate together with the rotor and extending parallel to the central axis of the crankshaft; and an output gear located on the output shaft, the driving unit further comprising: a first transmission gear located on the transmission shaft and configured to engage the output gear, the first transmission gear including more cogs than the output gear does; a second transmission gear located on the transmission shaft; and a driven gear located on the crankshaft and configured to engage the second transmission gear, the driven gear including more cogs than the second transmission gear does, a first transmission gear located on the transmission shaft and configured to engage the output gear, the first transmission gear including more cogs than the output gear does; a second transmission gear located on the transmission shaft; and a driven gear located on the crankshaft and configured to engage the second transmission gear, the driven gear including more cogs than the second transmission gear does, wherein, if out of the motor, first transmission gear and driven gear, a first component is a component with the largest diameter as viewed in an axial direction of the crankshaft, a second component is a component with the second largest diameter, and a third component is a component with the smallest diameter, characterized by a substrate as a printed circuit board, including a mounting surface expanding in directions perpendicular to the central axis of the crankshaft, the substrate is located between the first and second components as measured in an axial direction of the crankshaft, wherein the substrate is located in the same position as the third component as measured in an axial direction of the crankshaft.