In regard to at least one of long separator sheets (12a, 12b), measurement is carried out to measure the amount of curl at an edge of the at least one of the long separator sheets (12a, 12b) while applying tension to the at least one of the long separator sheets (12a, 12b) in the lengthwise direction (MD) of the at least one of the long separator sheets (12a, 12b), the edge being parallel to the lengthwise direction (MD) of the at least one of the long separator sheets (12a, 12b). This makes it possible to provide a method of measuring the amount of curl in a separator, a slitting apparatus, and a curl amount measuring apparatus, each of which is capable of carrying out a highly accurate nondestructive measurement of the amount of curl without having to cut a sample from a long separator sheet.