A riding type mowing machine comprising: a mowing device that includes a plurality of blades mowing grass, a mower deck covering the blades, and a transmission mechanism rotating the blades, the mowing device being provided between a front wheel and a rear wheel of a travelling vehicle, the transmission mechanism being provided above the mower deck, a left cover body covering the left side portion of the transmission mechanism and a right cover body covering the right side portion of the transmission mechanism being provided, a front portion of the mower deck being provided with an eject guide body forming an upper portion of an eject passage from which grass mown by the blades is ejected rightwardly, the eject guide body being formed in a posture inclined rightward and upwardly, an upper surface of a rear portion of the left cover body being formed as a flat surface with a left end portion and a rear end portion to be respectively bent downwardly, an upper surface of a rear portion of the right cover body being formed as a flat surface with a right end portion and a rear end portion to be respectively bent downwardly, and wherein a front portion of the left cover body and a front portion of the right cover body are detachably attached to the eject guide body in a posture along an upper surface thereof; characterized in that a center cover body (14, 15) covering a center portion of the transmission mechanism is provided between the left cover body and the right cover body, and wherein a rear portion of the center cover body (14, 15) is formed as a flat surface while a front portion thereof is detachably attached to the eject guide body in a posture along the upper surface thereof, the left cover body and the right cover body are formed so as to be thicker than the center cover body (14, 15), and wherein the center cover body (14, 15) is in close contact with the left cover body and the right cover body. a mowing device that includes a plurality of blades mowing grass, a mower deck covering the blades, and a transmission mechanism rotating the blades, the mowing device being provided between a front wheel and a rear wheel of a travelling vehicle, the transmission mechanism being provided above the mower deck, a left cover body covering the left side portion of the transmission mechanism and a right cover body covering the right side portion of the transmission mechanism being provided, a front portion of the mower deck being provided with an eject guide body forming an upper portion of an eject passage from which grass mown by the blades is ejected rightwardly, the eject guide body being formed in a posture inclined rightward and upwardly, an upper surface of a rear portion of the left cover body being formed as a flat surface with a left end portion and a rear end portion to be respectively bent downwardly, an upper surface of a rear portion of the right cover body being formed as a flat surface with a right end portion and a rear end portion to be respectively bent downwardly, and wherein a front portion of the left cover body and a front portion of the right cover body are detachably attached to the eject guide body in a posture along an upper surface thereof; characterized in that a center cover body (14, 15) covering a center portion of the transmission mechanism is provided between the left cover body and the right cover body, and wherein a rear portion of the center cover body (14, 15) is formed as a flat surface while a front portion thereof is detachably attached to the eject guide body in a posture along the upper surface thereof, the left cover body and the right cover body are formed so as to be thicker than the center cover body (14, 15), and wherein the center cover body (14, 15) is in close contact with the left cover body and the right cover body.