PURPOSE: To achieve a high degree of cleaning of an extracting device by making a bottom wall constituted of sectoral pieces which are bent by the action of a device being inserted into a test tube through the bottom wall of a sealing plug and are returnable to their initial position simultaneously when the device is pulled out of the test tube through the bottom wall.

CONSTITUTION: The sealing plug 1 of the test tube is so constituted that an opening part is sealed by providing a small pan-shape, protruding a circular collar 11 at the upper part, further, providing the horizontal bottom wall 12 and inserting the collar 11 into the opening part of the test tube 3 until the collar 11 hits on the peripheral edge part 31 of the opening of the test tube 3. Then, the horizontal bottom wall 12 is constituted of plural elastically deformed sectoral pieces which are bent by the action of the device 43 being inserted into the test tube 3 through the bottom wall 12 so as to introduce a liq. into the test tube 3 or extract the liq. from the inside of the test tube 3 and are returnable to their initial position simultaneously when the device 43 is pulled out of the test tube 3 through the bottom wall 12. This sealing plug is capable of executing a high degree of cleaning of the extracting device and a sufficient sealing of the liq. charged into the test tube.