a of the polygon mirror 5 of this embodiment is uniform for all the deflection surfaces of the polygon mirror 5 so that the bd sensor 11 constantly produces its output without fluctuations as a result it can provide high definition printing and effectively avoid any printing slippage in the main scanning direction fig1 is an enlarged schematic view of the bd mirror the mirror holding member and its vicinity of the second embodiment of light scanning optical system according to the invention this embodiment differs from the above described first embodiment in that the luminous flux delimiting means of this embodiment comprises a bd mirror holding member for holding the bd mirror otherwise this embodiment is identical with the first embodiment particularly in terms of its optical effects referring to fig1 reference symbol 28 denotes the luminous flux delimiting means comprising a bd mirror holding member bd mirror holder for holding the bd mirror and delimiting the width of the luminous flux entering the bd sensor not shown reference symbol 18 in fig1 denotes the bd mirror of this embodiment which is similar to that of fig1 or fig2 in this embodiment the width of the luminous flux and hence the quantity of light entering the bd sensor is substantially determined by the size of the aperture of the bd mirror holder 28 additionally in this embodiment the quantity of light overflowing the deflection surface 5 a of the light deflector 5 is small relative to the quantity of light delimited by the bd mirror holder 28 with the above described arrangement the quantity of light arriving to the.