The present invention relates to a sealing device for a secondary battery. The sealing device is configured to seal a battery case accommodating an electrode assembly therein to manufacture the secondary battery, the sealing device including a sealing block configured to press a sealing portion of the battery case, a flatness adjusting part configured to adjust a flatness of a pressing surface of the sealing block, and a heater part configured to transfer heat to the sealing block to thermally fuse the sealing portion of the battery case when the sealing portion is pressed by the sealing block.