A flow passage partition structure comprising: a flow passage member including a flow passage space portion through which fluid flows; a partition member partitioning the flow passage space portion into a first flow passage and a second flow passage; and the partition member comprising: a first plate being inserted into the flow passage space portion from one opening end portion of the flow passage member, and being in contact with a step portion provided at a boundary between the first flow passage and the second flow passage at an innermost side in an insertion direction of the first plate; a second plate being inserted into the flow passage space portion after the first plate is inserted into the flow passage space portion, the second plate being in contact with the first plate in such a manner that the second plate overlaps the first plate in a state where the second plate is press-fitted in an inner circumferential surface of the flow passage space portion; the first plate including an outer circumferential surface that is out of contact with the inner circumferential surface of the flow passage space portion; and the first plate, the second plate, and the inner circumferential surface of the flow passage space portion forming a space portion that corresponds to a foreign material storage portion. a flow passage member including a flow passage space portion through which fluid flows; a partition member partitioning the flow passage space portion into a first flow passage and a second flow passage; and the partition member comprising: a first plate being inserted into the flow passage space portion from one opening end portion of the flow passage member, and being in contact with a step portion provided at a boundary between the first flow passage and the second flow passage at an innermost side in an insertion direction of the first plate; a second plate being inserted into the flow passage space portion after the first plate is inserted into the flow passage space portion, the second plate being in contact with the first plate in such a manner that the second plate overlaps the first plate in a state where the second plate is press-fitted in an inner circumferential surface of the flow passage space portion; the first plate including an outer circumferential surface that is out of contact with the inner circumferential surface of the flow passage space portion; and the first plate, the second plate, and the inner circumferential surface of the flow passage space portion forming a space portion that corresponds to a foreign material storage portion. a first plate being inserted into the flow passage space portion from one opening end portion of the flow passage member, and being in contact with a step portion provided at a boundary between the first flow passage and the second flow passage at an innermost side in an insertion direction of the first plate; a second plate being inserted into the flow passage space portion after the first plate is inserted into the flow passage space portion, the second plate being in contact with the first plate in such a manner that the second plate overlaps the first plate in a state where the second plate is press-fitted in an inner circumferential surface of the flow passage space portion; the first plate including an outer circumferential surface that is out of contact with the inner circumferential surface of the flow passage space portion; and the first plate, the second plate, and the inner circumferential surface of the flow passage space portion forming a space portion that corresponds to a foreign material storage portion.