An image forming apparatus comprising: an image forming portion (41, 42, 91) configured to form toner images on recording materials having a first size and a second size smaller than the first size with respect to a longitudinal direction; a fixing unit configured to form a nip in which the toner image is fixed by nipping and feeding the recording material, wherein said fixing unit is capable of fixing the toner image on the recording material having the first size which is a maximum fixable size of the recording material, wherein said fixing unit includes, a heating unit including an elongated heater extending in the longitudinal direction perpendicular to a feeding direction of the recording material, air blowing means configured to blow air toward one end portion and the other end portion of said heating unit with respect to the longitudinal direction when the recording material having the second size is inserted into said fixing unit, a first opening through which air blown from said air blowing means toward said one end portion of said heating unit passes, a first shielding member configured to change an opening width of said first opening with respect to the longitudinal direction, a second opening through which air blown from said air blowing means toward said the other end portion of said heating unit passes, a second shielding member configured to change an opening width of said second opening with respect to the longitudinal direction, detecting means configured to detect a deviation amount of the recording material in a widthwise direction with respect to the feeding direction of the recording material, and a controller configured to control, on the basis of an output of said detecting means and size information of the recording material, the opening widths of said first and second openings (116c, 116b) so as to be different from each other by moving said first and second shielding members (112, 211), respectively, characterized in that the image forming apparatus further comprises a driving motor as a common driving source configured to move said first and second shielding members (112, 211), wherein said first shielding member includes a first shielding portion configured to shield said first opening, said first shielding portion being fixed to a first driven portion configured to receive drive from said driving source, and wherein said second shielding member includes a second shielding portion configured to shield said second opening, said second shielding portion being movable by a predetermined amount in a movement direction relative to a second driven portion configured to receive drive from said driving source. an image forming portion (41, 42, 91) configured to form toner images on recording materials having a first size and a second size smaller than the first size with respect to a longitudinal direction; a fixing unit configured to form a nip in which the toner image is fixed by nipping and feeding the recording material, wherein said fixing unit is capable of fixing the toner image on the recording material having the first size which is a maximum fixable size of the recording material, wherein said fixing unit includes, a heating unit including an elongated heater extending in the longitudinal direction perpendicular to a feeding direction of the recording material, air blowing means configured to blow air toward one end portion and the other end portion of said heating unit with respect to the longitudinal direction when the recording material having the second size is inserted into said fixing unit, a first opening through which air blown from said air blowing means toward said one end portion of said heating unit passes, a first shielding member configured to change an opening width of said first opening with respect to the longitudinal direction, a second opening through which air blown from said air blowing means toward said the other end portion of said heating unit passes, a second shielding member configured to change an opening width of said second opening with respect to the longitudinal direction, detecting means configured to detect a deviation amount of the recording material in a widthwise direction with respect to the feeding direction of the recording material, and a controller configured to control, on the basis of an output of said detecting means and size information of the recording material, the opening widths of said first and second openings (116c, 116b) so as to be different from each other by moving said first and second shielding members (112, 211), respectively, the image forming apparatus further comprises a driving motor as a common driving source configured to move said first and second shielding members (112, 211), wherein said first shielding member includes a first shielding portion configured to shield said first opening, said first shielding portion being fixed to a first driven portion configured to receive drive from said driving source, and wherein said second shielding member includes a second shielding portion configured to shield said second opening, said second shielding portion being movable by a predetermined amount in a movement direction relative to a second driven portion configured to receive drive from said driving source.