A reel unit for a spinning reel has a casing and a lid portion. The casing has an opening formed in its side portion, a mechanism accommodating space formed inside, and a rotation support portion for supporting a rotation shaft (pinion gear) of the rotor. The lid portion has a cover portion and a mounting leg portion. The cover portion is mounted detachably to the casing to cover the opening of the casing. The cover portion is formed unitarily with a second flange portion so as to define a space in the interior thereof. The mounting leg portion extends from the cover portion and to the front end of which a fishing rod can be mounted. In this reel unit, the weight of the lid portion is reduced at low cost while precision and relative strength are maintained.