A spinning reel configured to reel out a fishing line in a forward direction, comprising: a handle; a reel unit supporting the handle in a rotatable state, the reel unit having a fishing rod attachment portion and a mount portion (2e, 102e), the fishing rod attachment portion allowing a fishing rod to be attached thereto, the mount portion (2e, 102e) having a circular shape, the mount portion (2e, 102e) being disposed in a front part of the reel unit, the mount portion having a through hole in a center thereof, the through hole extending in the forward direction; a rotor having an attachment portion having a tubular shape, the rotor being rotatably mounted to the reel unit; a spool on which the fishing line is wound by the rotor, the spool being mounted to the reel unit to be reciprocated in a back-and-forth direction being parallel to the forward direction; a rotor drive mechanism including a drive gear and a pinion gear, the drive gear having face gear teeth, the drive gear being configured to be rotated in conjunction with rotation of the handle, the pinion gear having a tubular shape, the pinion gear being coupled at a front portion thereof to the attachment portion of the rotor to be unitarily rotatable with the attachment portion, the pinion gear being disposed in the through hole, the pinion gear being meshed with the drive gear; and an anti-reverse mechanism (50, 150) configured to prevent the rotor from rotating in a fishing-line releasing direction, the anti-reverse mechanism (50, 150) including a one-way clutch assembly (51, 151) being detachably attached to the mount portion (2e, 102e) of the reel unit, the rotor drive mechanism including a seal member, wherein the seal member is configured to seal a clearance between the pinion gear and the through hole while being disposed at a rear end of the one-way clutch assembly (51, 151) in the back-and-forth direction, characterised in that the pinion gear includes an attachment groove annularly formed on an outer peripheral portion thereof, the seal member is attached to the attachment groove, and the seal member is in contact with an inner peripheral surface of the through hole, and in that the seal member is configured to be rotated together with the pinion gear. a handle; a reel unit supporting the handle in a rotatable state, the reel unit having a fishing rod attachment portion and a mount portion (2e, 102e), the fishing rod attachment portion allowing a fishing rod to be attached thereto, the mount portion (2e, 102e) having a circular shape, the mount portion (2e, 102e) being disposed in a front part of the reel unit, the mount portion having a through hole in a center thereof, the through hole extending in the forward direction; a rotor having an attachment portion having a tubular shape, the rotor being rotatably mounted to the reel unit; a spool on which the fishing line is wound by the rotor, the spool being mounted to the reel unit to be reciprocated in a back-and-forth direction being parallel to the forward direction; a rotor drive mechanism including a drive gear and a pinion gear, the drive gear having face gear teeth, the drive gear being configured to be rotated in conjunction with rotation of the handle, the pinion gear having a tubular shape, the pinion gear being coupled at a front portion thereof to the attachment portion of the rotor to be unitarily rotatable with the attachment portion, the pinion gear being disposed in the through hole, the pinion gear being meshed with the drive gear; and an anti-reverse mechanism (50, 150) configured to prevent the rotor from rotating in a fishing-line releasing direction, the anti-reverse mechanism (50, 150) including a one-way clutch assembly (51, 151) being detachably attached to the mount portion (2e, 102e) of the reel unit, the rotor drive mechanism including a seal member, wherein the seal member is configured to seal a clearance between the pinion gear and the through hole while being disposed at a rear end of the one-way clutch assembly (51, 151) in the back-and-forth direction, characterised in that the pinion gear includes an attachment groove annularly formed on an outer peripheral portion thereof, the seal member is attached to the attachment groove, and the seal member is in contact with an inner peripheral surface of the through hole, and in that the seal member is configured to be rotated together with the pinion gear.