An oil mist separator includes a case. The case includes an inlet, an outlet, a gas passage, and an oil discharge portion. The case includes a first member located in an upper portion of the case, a second member located in a lower portion of the case, and a third member partitioning the first member and the second member. The first member, the second member, and the third member are welded to each other in a stacked state. The gas passage includes an upper passage defined by the first member and the third member, a lower passage defined by the second member and the third member, and a connection passage. The connection passage connects the upper passage to the lower passage at a portion in the case corresponding to an end of the case on one side.