A blowby gas treatment device comprising: an internal combustion engine including an engine gas passage, the engine gas passage being integral with the internal combustion engine; an intake manifold including an intake passage, an intake manifold gas passage, and a communication hole, the intake manifold gas passage being integral with the intake manifold; a blowby gas passage being configured to connect an inside of the internal combustion engine to the intake passage, the blowby gas passage being configured such that blowby gas passes through the blowby gas passage; a flow control valve provided in the blowby gas passage, the flow control valve being configured to adjust an amount of the blowby gas; and a lid configured to open and close the communication hole, the communication hole being configured to connect an outside of the intake manifold to a part of the intake manifold gas passage, the part being located on a downstream side of the flow control valve in a gas flow direction, and the communication hole opening the intake manifold gas passage to an atmosphere, characterized in that: the lid includes a first side, a first through hole, and a slit, in which the slit includes a second through hole, and the first side is one of long sides of the lid, the first through hole and the second through hole are provided at a given distance from each other, the slit extends to the first side from the second through hole, the slit extends to an intersection point at which a raceway of a circumference of a circle and the first side intersect with each other, when the circle is drawn in which the distance from the first through hole to the second through hole serves as a radius, and the lid is configured to be fastened to the intake manifold by a first external thread inserted into the first through hole and a second external thread inserted into the second through hole when closing the communication hole. an internal combustion engine including an engine gas passage, the engine gas passage being integral with the internal combustion engine; an intake manifold including an intake passage, an intake manifold gas passage, and a communication hole, the intake manifold gas passage being integral with the intake manifold; a blowby gas passage being configured to connect an inside of the internal combustion engine to the intake passage, the blowby gas passage being configured such that blowby gas passes through the blowby gas passage; a flow control valve provided in the blowby gas passage, the flow control valve being configured to adjust an amount of the blowby gas; and a lid configured to open and close the communication hole, the communication hole being configured to connect an outside of the intake manifold to a part of the intake manifold gas passage, the part being located on a downstream side of the flow control valve in a gas flow direction, and the communication hole opening the intake manifold gas passage to an atmosphere, characterized in that: the lid includes a first side, a first through hole, and a slit, in which the slit includes a second through hole, and the first side is one of long sides of the lid, the first through hole and the second through hole are provided at a given distance from each other, the lid includes a first side, a first through hole, and a slit, in which the slit includes a second through hole, and the first side is one of long sides of the lid, the first through hole and the second through hole are provided at a given distance from each other, the slit extends to the first side from the second through hole, the slit extends to an intersection point at which a raceway of a circumference of a circle and the first side intersect with each other, when the circle is drawn in which the distance from the first through hole to the second through hole serves as a radius, and the lid is configured to be fastened to the intake manifold by a first external thread inserted into the first through hole and a second external thread inserted into the second through hole when closing the communication hole.