A work machine, comprising: a lower propelling body; and a machine body mounted on the lower propelling body, the machine body including an engine room in a rear portion thereof, wherein the machine body includes: a bottom plate that forms a bottom surface of the engine room and has an air outlet formed therein; a fan that is installed in the engine room to generate an air flow flowing in a left and right direction of the machine body in the engine room; an engine that has a drive shaft and is laterally disposed in the engine room such that the drive shaft extends in the left and right direction of the machine body; and an exhaust air duct that is installed in the engine room to guide the air flow to the air outlet, characterized in that the air outlet includes a major opening portion that extends along a lower portion of a rear portion of the engine in the left and right direction of the machine body, and a wide portion that is disposed with the major opening portion side by side in the left and right direction of the machine body so as to be connected to the major opening portion, wherein the wide portion is disposed only at one of the sides of the major opening portion in the left and right direction, the wide portion including a portion located immediately under the engine and having a greater width in a front and rear direction of the machine body than a width of the major opening portion in the front and rear direction of the machine body, the exhaust air duct includes a duct base portion that is installed on the bottom plate so as to cover the air outlet, the duct base portion including a lower opening that is provided at a lower end of the duct base portion to communicate with the air outlet, and an extension portion that extends upward from the duct base portion at a position behind the engine and has an upper opening that is provided at an upper end thereof and is open upward, the exhaust air duct defines a ventilation flue in the exhaust air duct, the ventilation flue communicating the upper opening with the lower opening between the upper opening and the lower opening, and the duct base portion includes a wide-portion covering portion that covers the wide portion at a position immediately under the engine, wherein a region extending from a front portion of the wide-portion covering portion to a specific position in the rear side is disposed in a space located immediately under the engine and the region covers a portion of the wide portion located immediately under the engine. a lower propelling body; and a machine body mounted on the lower propelling body, the machine body including an engine room in a rear portion thereof, wherein the machine body includes: a bottom plate that forms a bottom surface of the engine room and has an air outlet formed therein; a fan that is installed in the engine room to generate an air flow flowing in a left and right direction of the machine body in the engine room; an engine that has a drive shaft and is laterally disposed in the engine room such that the drive shaft extends in the left and right direction of the machine body; and an exhaust air duct that is installed in the engine room to guide the air flow to the air outlet, characterized in that the air outlet includes a major opening portion that extends along a lower portion of a rear portion of the engine in the left and right direction of the machine body, and a wide portion that is disposed with the major opening portion side by side in the left and right direction of the machine body so as to be connected to the major opening portion, wherein the wide portion is disposed only at one of the sides of the major opening portion in the left and right direction, the wide portion including a portion located immediately under the engine and having a greater width in a front and rear direction of the machine body than a width of the major opening portion in the front and rear direction of the machine body, the exhaust air duct includes a duct base portion that is installed on the bottom plate so as to cover the air outlet, the duct base portion including a lower opening that is provided at a lower end of the duct base portion to communicate with the air outlet, and an extension portion that extends upward from the duct base portion at a position behind the engine and has an upper opening that is provided at an upper end thereof and is open upward, the exhaust air duct defines a ventilation flue in the exhaust air duct, the ventilation flue communicating the upper opening with the lower opening between the upper opening and the lower opening, and the duct base portion includes a wide-portion covering portion that covers the wide portion at a position immediately under the engine, wherein a region extending from a front portion of the wide-portion covering portion to a specific position in the rear side is disposed in a space located immediately under the engine and the region covers a portion of the wide portion located immediately under the engine.