Provided is a separator including a first later consisting of a porous polyolefin and a secondary battery utilizing the separator. The first layer exhibits a temperature-increase convergence time equal to or longer than 2.9 s·m2/g and equal to or shorter than 5.7 s·m2/g when the first layer is irradiated with a microwave having a frequency of 2455 MHz and an output power of 1800 W after being dipped in N-methylpyrrolidone containing 3 wt % of water. A minimum height of a ball placed over the first layer and having a diameter of 14.3 mm and a weight of 11.9 g is equal to or more than 50 cm and equal to or less than 150 cm, the minimum height causing a split in the first layer when the ball freely falls on the first layer.