A core prevents or reduces misalignment of a battery separator wound around the core. The core has one or more grooves in the outer peripheral surface thereof. The grooves extend substantially in the width direction of the core. The grooves have a depth of 30 μm or greater, and the following condition (1) is satisfied: N/D>0.0025  (1) where D is the outer circumference in mm of the core and N is the number of the grooves in the outer peripheral surface.