A manufacturing method for a battery case lid including an explosion-proof valve, the manufacturing method comprising: an extending step of pushing an extending punch from a first side of a metal plate disposed over an extending die having a recess, to form a reduced thickness section constituting the explosion-proof valve, wherein the recess has an inner diameter greater than an outer diameter of the extending punch, an external corner portion of the recess and a top section on a side face of the extending punch have a peripheral portion arc-shaped in cross section, the top section being formed by an end of the extending punch facing the recess, a clearance (c) between a side face of the recess of the extending die and an outer periphery at the top section on the side face of the extending punch is set to be smaller than a thickness (t) of the metal plate, in the extending step, the extending punch is pushed while the metal plate is kept unrestrained, and the metal plate is pressed to be extended between a bottom face of the recess and a pressing face of the extending punch, to form the reduced thickness section and a cylindrical wall portion, characterized in that the extending step is followed by a bending step in which the reduced thickness section is pressed by a bending punch smaller in outer diameter than the reduced thickness section from a second side of the metal plate, to bend the cylindrical wall portion formed around the reduced thickness section, and wherein in the bending step in which the metal plate is placed on a bending die having a protruded portion with the second side of the metal plate up and with the reduced thickness section positioned over the protruded portion, then the bending punch is pressed downward against the reduced thickness section to the extent that the reduced thickness section contacts the protruded portion. an extending step of pushing an extending punch from a first side of a metal plate disposed over an extending die having a recess, to form a reduced thickness section constituting the explosion-proof valve, wherein the recess has an inner diameter greater than an outer diameter of the extending punch, an external corner portion of the recess and a top section on a side face of the extending punch have a peripheral portion arc-shaped in cross section, the top section being formed by an end of the extending punch facing the recess, a clearance (c) between a side face of the recess of the extending die and an outer periphery at the top section on the side face of the extending punch is set to be smaller than a thickness (t) of the metal plate, in the extending step, the extending punch is pushed while the metal plate is kept unrestrained, and the metal plate is pressed to be extended between a bottom face of the recess and a pressing face of the extending punch, to form the reduced thickness section and a cylindrical wall portion, characterized in that the extending step is followed by a bending step in which the reduced thickness section is pressed by a bending punch smaller in outer diameter than the reduced thickness section from a second side of the metal plate, to bend the cylindrical wall portion formed around the reduced thickness section, and wherein in the bending step in which the metal plate is placed on a bending die having a protruded portion with the second side of the metal plate up and with the reduced thickness section positioned over the protruded portion, then the bending punch is pressed downward against the reduced thickness section to the extent that the reduced thickness section contacts the protruded portion.