To measure the mass per unit area of an applied layer in a layered film while preventing a measurement error from being caused by thickness unevenness, a per-unit-area-mass measuring device (30) includes a light projector (31a) configured to project light having a center wavelength of 405 nm, a light projector (31b) configured to project light having a center wavelength of 850 nm, light receivers (32a, 32b) configured to receive light having been transmitted through a separator (12), and a control section (33) configured to calculate the mass per unit area of a heat-resistant layer (4) on the basis of the respective transmitted-light intensities of the light having a center wavelength of 405 nm and the light having a center wavelength of 850 nm.