An air cleaner for an internal combustion engine, comprising: a case including a case body with an opening, and an inlet duct formed integrally with the case body leading to the opening; a cap including an outlet, and an opening facing the opening of the case body and leading air to the outlet, and a filter element provided between the opening of the case body and the opening of the cap, wherein the case includes a first case molding and a second case molding, the first case molding includes an opening-side circumferential wall portion surrounding the opening of the case body, and a first inlet duct half in a semi-cylindrical shape, the second case molding includes a bottom wall of the case body, a bottom-wall-side circumferential wall portion formed continuously with the bottom wall, and a second inlet duct half in a semi-cylindrical shape extending continuously with the bottom-wall-side circumferential wall portion, the circumference of the opening-side circumferential wall portion and the circumference of the bottom-wall-side circumferential wall portion are joined to each other to form the case body, and the circumference of the fist inlet duct half and the circumference of the second inlet duct half are joined to each other to form the inlet duct, the first case molding is formed as a single piece, the second case molding is formed as a single piece, the first inlet duct half extends continuously with the opening-side circumferential wall portion, such that the inlet duct includes a connection portion, which extends along a circumferential direction of the opening of the case body characterized in that the connection portion is formed over the entire width of one face of said opening-side circumferential wall portion of the case body, the first case molding is provided with a first resonator half in a semi-cylindrical shape, the second case molding is provided with a second resonator half in a semi-cylindrical shape, and the circumference of the first resonator half and the circumference of the second resonator half are joined to each other to form a resonator of a Helmholtz type. a case including a case body with an opening, and an inlet duct formed integrally with the case body leading to the opening; a cap including an outlet, and an opening facing the opening of the case body and leading air to the outlet, and a filter element provided between the opening of the case body and the opening of the cap, wherein the case includes a first case molding and a second case molding, the first case molding includes an opening-side circumferential wall portion surrounding the opening of the case body, and a first inlet duct half in a semi-cylindrical shape, an opening-side circumferential wall portion surrounding the opening of the case body, and a first inlet duct half in a semi-cylindrical shape, the second case molding includes a bottom wall of the case body, a bottom-wall-side circumferential wall portion formed continuously with the bottom wall, and a second inlet duct half in a semi-cylindrical shape extending continuously with the bottom-wall-side circumferential wall portion, a bottom wall of the case body, a bottom-wall-side circumferential wall portion formed continuously with the bottom wall, and a second inlet duct half in a semi-cylindrical shape extending continuously with the bottom-wall-side circumferential wall portion, the circumference of the opening-side circumferential wall portion and the circumference of the bottom-wall-side circumferential wall portion are joined to each other to form the case body, and the circumference of the fist inlet duct half and the circumference of the second inlet duct half are joined to each other to form the inlet duct, the first case molding is formed as a single piece, the second case molding is formed as a single piece, the first inlet duct half extends continuously with the opening-side circumferential wall portion, such that the inlet duct includes a connection portion, which extends along a circumferential direction of the opening of the case body characterized in that the connection portion is formed over the entire width of one face of said opening-side circumferential wall portion of the case body, the first case molding is provided with a first resonator half in a semi-cylindrical shape, the second case molding is provided with a second resonator half in a semi-cylindrical shape, and the circumference of the first resonator half and the circumference of the second resonator half are joined to each other to form a resonator of a Helmholtz type.