The Trocar structure for abdominal surgery comprises a cannula having at one end a guide element provided with a valve for maintaining the positive intracavitary pressure and a shaft having, at one end a head and at the opposite end a tip, advantageously the valve is made of a single piece of silicone and has first elastically deformable sealing means for maintaining the positive intracavitary pressure during the passage of the shaft and of a surgical instrument through the cannula and second elastically deformable sealing means engaging the guide element.