A writing instrument having an automatic sealing structure that may reduce an entire diameter of the writing instrument due to a decrease in a size of a ball shutter, thus to improve grip feeling of a user. The writing instrument having an automatic sealing structure includes: a body case having a front opening formed in one end thereof; a knock part coupled to the other end of the body case so as to make a nib protrude from or be retracted into the front opening of the body case by a pushing operation thereof; a cartridge inserted into the body case together with a spring in a state in which the knock part and the nib are coupled to both ends thereof, respectively; at least one O-ring fixedly fitted in the body case; a holder fixedly fitted in the body case so that one end thereof is covered by the O-ring; a ball shutter pivotally coupled to the holder to allow the nib to protrude or be sealed according to the pivoting thereof; and a link which is slidably fitted in the holder, and has a cutaway piece formed by protruding a circumferential portion of a part of a hollow column end thereof to allow the ball shutter to be pivoted.