A mail feeding device comprising: a pickup member including suction holes formed therein, and configured to run along one of accumulated mail items in a direction in which the one mail item is picked up, the one mail item being accumulated earliest and positioned at a pickup position; a negative-pressure chamber including an opening which opposes the pickup position with the pickup member interposed therebetween, the negative-pressure chamber applying negative pressure, via the suction holes, to the one mail item positioned at the pickup position, thereby making the one mail item to be held by the pickup member; drawing means which draws air from the negative-pressure chamber; a suction pipe connecting the drawing means to the negative-pressure chamber; a conveyance section (8a, 8b) configured to receive the one mail item held on the pickup member by the negative pressure and picked up from the pickup position, and to convey the picked up mail item; a detector section configured to detect whether the picked up mail item has been transferred to a conveyance path; a pressure adjustment unit (13, 16, 30, 30', 40, 50, 60) configured to introduce air into the negative-pressure chamber, from which air was drawn by the drawing means, to increase internal pressure of the negative-pressure chamber toward atmospheric pressure; and a controller configured to cause the pressure adjustment unit (13, 16, 30, 30', 40, 50, 60) to increase the internal pressure of the negative-pressure chamber toward atmospheric pressure after the detector section detects that the picked up mail item has been transferred to the conveyance path, wherein the pressure adjustment unit (13, 16, 30, 30', 40, 50, 60) includes an air supply unit (13, 16) configured to positively introduce air into the negative-pressure chamber, characterized in that the pressure adjustment unit (13, 16, 30, 30', 40, 50, 60) further includes: an air supply pipe (32, 42) provided separately from the suction pipe, the air supply pipe (32, 42) connecting the air supply unit (13, 16) to the negative-pressure chamber, and an on/off valve (34, 44) provided across the air supply pipe (32, 42), the controller executing on/off control of the on/off valve (34, 44). a pickup member including suction holes formed therein, and configured to run along one of accumulated mail items in a direction in which the one mail item is picked up, the one mail item being accumulated earliest and positioned at a pickup position; a negative-pressure chamber including an opening which opposes the pickup position with the pickup member interposed therebetween, the negative-pressure chamber applying negative pressure, via the suction holes, to the one mail item positioned at the pickup position, thereby making the one mail item to be held by the pickup member; drawing means which draws air from the negative-pressure chamber; a suction pipe connecting the drawing means to the negative-pressure chamber; a conveyance section (8a, 8b) configured to receive the one mail item held on the pickup member by the negative pressure and picked up from the pickup position, and to convey the picked up mail item; a detector section configured to detect whether the picked up mail item has been transferred to a conveyance path; a pressure adjustment unit (13, 16, 30, 30', 40, 50, 60) configured to introduce air into the negative-pressure chamber, from which air was drawn by the drawing means, to increase internal pressure of the negative-pressure chamber toward atmospheric pressure; and a controller configured to cause the pressure adjustment unit (13, 16, 30, 30', 40, 50, 60) to increase the internal pressure of the negative-pressure chamber toward atmospheric pressure after the detector section detects that the picked up mail item has been transferred to the conveyance path, wherein the pressure adjustment unit (13, 16, 30, 30', 40, 50, 60) includes an air supply unit (13, 16) configured to positively introduce air into the negative-pressure chamber, characterized in that the pressure adjustment unit (13, 16, 30, 30', 40, 50, 60) further includes: an air supply pipe (32, 42) provided separately from the suction pipe, the air supply pipe (32, 42) connecting the air supply unit (13, 16) to the negative-pressure chamber, and an on/off valve (34, 44) provided across the air supply pipe (32, 42), the controller executing on/off control of the on/off valve (34, 44). an air supply pipe (32, 42) provided separately from the suction pipe, the air supply pipe (32, 42) connecting the air supply unit (13, 16) to the negative-pressure chamber, and an on/off valve (34, 44) provided across the air supply pipe (32, 42), the controller executing on/off control of the on/off valve (34, 44).