A trocar device includes a main part that has a first and a second end. A shaft tube is connected to the second end of the main part. A through-opening for receiving an instrument extends from the first end of the main part to the end of the shaft tube that faces away from the main part. A sealing unit is disposed in the main part and has an elastic sealing element having a hollow-cylindrical wall, which divides the through-opening into a first region that runs from the sealing unit as far as the first end of the main body, and into a second region that runs from the sealing unit as far as the end of the shaft tube that faces away from the main part. When the instrument has been properly inserted in the through-opening, the sealing element seals off the two regions from each other.