Provided is a tight-sealing cap for the liquid drug-expelling part of a syringe that not only minimizes the space that creates reduced pressure, which is a cause of leakage, but is also easy to remove because it is easily twisted off, easily deformable and easily admits air for releasing the vacuum when removed, and that can prevent leaking of a liquid drug from a luer nozzle. The cap is made of an elastic material, and the following means are used in the cap, which is configured from a thick upper surface part, an outer tubular part that is formed below the periphery thereof, and an inner tubular part that is formed inside the same. First, the inner tubular part is formed so that the tip of the luer nozzle can be inserted tightly into the same, and the luer nozzle can be tightly sealed at the site of contact thereof. Second, the outer tubular part is formed so that it contacts the luer lock closely therein and can seal the luer lock tightly at the site of contact thereof. Third, the length of the site of contact between the outer tubular part and the luer lock is shorter than the length of the site of contact between the inner tubular part and the luer nozzle.