To prevent deterioration in the life of a belt caused by difficult exhaust of heat in the inside of a covering body covering a belt transmission in a walking type agricultural working machine.

The covering body 40 is composed so that a belt part A covering the belt transmission is integral with an exhaust pipe part B covering an exhaust pipe of an engine. A communicating opening 45 is provided between the belt part A and the exhaust pipe part B so that air flows between both. A part of cooling air of the engine collides with the inside of a side plate 43 of the exhaust pipe part B, passes through the communicating opening 45, enters the belt part A, flows in the direction of an arrow A2 according to the rotation of the belt, passes through the communicating opening 45 and returns to the exhaust pipe part B. Thereby, an air conduit passage A2 in which the cooling air of the engine is circulated and discharged is formed in the interior of the covering body 40. The belt can be cooled with the cooling air of the engine flowing along the air conduit passage A2.