In a slide position detecting device for a vehicle seat, an angle between a bottom plate portion and a side plate portion of an attachment bracket is set to a maximum angle in a range of an angle between a bottom wall portion and a side wall portion of a seat-side member; rigidity maintaining the angle between the bottom plate portion and the side plate portion is set to be lower than rigidity maintaining the angle between the bottom wall portion and the side wall portion; and when the side plate portion is fixed to the side wall portion, the bottom plate portion contacts the bottom wall portion or a top face portion of an upper rail, and is fixed in a state where the angle between the bottom plate portion and the side plate portion coincides with the angle between the bottom wall portion and the side wall portion.