A drive device (100; 100C) for driving a driven gear situated on a movable portion of a power generator device which generates electric power under a change in a flow direction of fluid, the drive device (100; 100C) comprising: a drive gear configured to mesh with the driven gear, the drive gear being formed with a through-hole; and a drive portion including a rotary shaft portion inserted into the through-hole of the drive gear and a drive source being configured to generate a driving force for rotating the rotary shaft portion, the rotary shaft portion being connected to the drive gear (200Bby spline connection or key connection, the driving force being transmitted from the rotary shaft portion to the drive gear, wherein the drive gear includes an inner wall surface which forms a contour of the through-hole and which comprises a first inner wall surface portion, a second inner wall surface portion and a third inner wall surface portion situated between the first and the second inner wall surface portions, and wherein the rotary shaft portion includes a shaft which extends through the through-hole, and a plate member connected to a distal end surface of the shaft, and wherein the rotary shaft portion includes a first press fit portion which comes into contact with and is pressed against the first inner wall surface portion of the drive gear in a direction orthogonal to a rotational axis of the rotary shaft portion on a first imaginary plane (PPI; 221) orthogonal to the rotational axis, wherein the shaft includes the first press fit portion (333; 342) formed at a position away from the distal end surface; characterized by a second press fit portion (331; 350) which comes into contact with and is pressed against the inner wall surface (210; 241; 251; 251B) of the drive gear on a second imaginary plane (PP2; 222) away from the first imaginary plane (PP1; 221) in an extending direction of the rotational axis, and an intermediate portion situated between the first press fit portion (333, 342) and the second press fit portion (331; 350), wherein a pressing force between the inner wall surface (210; 241; 251; 251 B) and the intermediate portion is lower than a pressing force between the inner wall surface (210; 241; 251; 251 B) and the first press fit portion (333, 342), and a pressing force between the inner wall surface (210; 241; 251; 251 B) and the second press fit portion (331; 350), and wherein a recessed portion is formed by the second inner wall surface portion in cooperation with the third inner wall surface portion and the plate member is situated in the recessed portion, and a diameter of the plate member is slightly larger than the inner diameter of the second inner wall surface portion of the recessed portion and the plate member projects in a radial direction of the shaft from the distal end surface to form the second press fit portion. a drive gear configured to mesh with the driven gear, the drive gear being formed with a through-hole; and a drive portion including a rotary shaft portion inserted into the through-hole of the drive gear and a drive source being configured to generate a driving force for rotating the rotary shaft portion, the rotary shaft portion being connected to the drive gear (200Bby spline connection or key connection, the driving force being transmitted from the rotary shaft portion to the drive gear, wherein the drive gear includes an inner wall surface which forms a contour of the through-hole and which comprises a first inner wall surface portion, a second inner wall surface portion and a third inner wall surface portion situated between the first and the second inner wall surface portions, and wherein the rotary shaft portion includes a shaft which extends through the through-hole, and a plate member connected to a distal end surface of the shaft, and wherein the rotary shaft portion includes a first press fit portion which comes into contact with and is pressed against the first inner wall surface portion of the drive gear in a direction orthogonal to a rotational axis of the rotary shaft portion on a first imaginary plane (PPI; 221) orthogonal to the rotational axis, wherein the shaft includes the first press fit portion (333; 342) formed at a position away from the distal end surface; characterized by a second press fit portion (331; 350) which comes into contact with and is pressed against the inner wall surface (210; 241; 251; 251B) of the drive gear on a second imaginary plane (PP2; 222) away from the first imaginary plane (PP1; 221) in an extending direction of the rotational axis, and an intermediate portion situated between the first press fit portion (333, 342) and the second press fit portion (331; 350), wherein a pressing force between the inner wall surface (210; 241; 251; 251 B) and the intermediate portion is lower than a pressing force between the inner wall surface (210; 241; 251; 251 B) and the first press fit portion (333, 342), and a pressing force between the inner wall surface (210; 241; 251; 251 B) and the second press fit portion (331; 350), and wherein a recessed portion is formed by the second inner wall surface portion in cooperation with the third inner wall surface portion and the plate member is situated in the recessed portion, and a diameter of the plate member is slightly larger than the inner diameter of the second inner wall surface portion of the recessed portion and the plate member projects in a radial direction of the shaft from the distal end surface to form the second press fit portion.