A separator 100 is disposed between a positive electrode and a negative electrode in a lead acid storage battery including the positive electrode and the negative electrode, in which the separator 100 contains a glass fiber and an organic binder, the separator 100 includes a first layer 110a that is in contact with the positive electrode, and a second layer 110b that is in contact with the negative electrode, an average pore diameter of the first layer 110a is larger than an average pore diameter of the second layer 110b, and a thickness of the first layer 110a is equal to or less than the half of the overall thickness of the separator 100.