There is provided a novel regulating member that ensures easy manufacturing and does not adversely affect cooling water and environment. A spacer 6 is inserted from an opening 30 of a cooling water flow passage 3 into the cooling water flow passage 3 to be disposed. The cooling water flow passage 3 is disposed in a cylinder block 1 in an internal combustion engine. The spacer 6 includes a supporting member 7 with rigidity formed into a shape configured to be disposed in the cooling water flow passage 3 and a regulating portion 8 supported by the supporting member 7. The regulating portion 8 regulates a flow of cooling water. The regulating portion 8 includes a cellulose-based sponge 81. The cellulose-based sponge 81 is restorable from a compressed state through a contact with the cooling water w.