An intake passage is formed by connecting an upper case 10 and a lower case 20 of an intake manifold. The lower case 10 has a recessed portion 50 on part of an inner surface 17a of a side wall 17. The recessed portion 50 has a deep surface 53 to which a negative pressure outlet port 42 opens, and an opening 51 opened upwards is provided in an upper portion of the recessed portion 50 in a position which opposes to the upper case 20 in a vertical direction A0. The upper case 20 has a projecting portion 60 which extends further downwards towards the negative pressure outlet port 42 than mating surfaces 10a, 20a and projects into the recessed portion 50 through an opening 52. The projecting portion 60 is positioned between the negative pressure outlet port 42 and the inner surface 17a in a horizontal direction A1.