A spinning reel master gear assembly is rotatably attachable to a reel unit of a spinning reel. The spinning reel master gear assemble includes a master gear shaft being rotatably attachable to the reel unit, and a master gear fixed to the master gear shaft in an integrally rotatable state. The master gear shaft includes an attachment portion. The attachment portion has a male threaded portion at least partially on an outer peripheral surface thereof. The master gear includes a female threaded portion screwed onto the male threaded portion, and a centered portion including an axis appropriately aligned by the attachment portion.
