A method of manufacturing a liquid cooling jacket in which a jacket body and a sealing body are joined together by friction stirring to form the liquid cooling jacket being characterised by the followings steps: a preparation step of forming the jacket body that includes a bottom part, a peripheral wall part rising from a peripheral edge of the bottom part, a stepped bottom surface formed at a position below by one step from an end face of the peripheral wall part, a stepped side surface rising from the stepped bottom surface, and a support part erected on the bottom part and formed with a projecting part on an end face thereof, and forming a sealing body that is formed with a hole into which the projecting part is inserted and that seals an opening of the jacket body; a sealing body disposing step of disposing the sealing body on the jacket body to make the stepped side surface abut on a side surface of the sealing body to form a first abutment portion, and to make an outer peripheral surface of the projecting part abut on a wall of the hole to form a second abutment portion; an auxiliary member disposing step of disposing an auxiliary member along the first abutment portion; a joining step of moving a rotary tool along the first abutment portion and the second abutment portion to perform friction stirring; and a removing step of removing the auxiliary member on which burrs are formed, wherein the rotary tool is moved along the first abutment portion with only a stirring pin of the rotary tool being in contact with the stepped side surface, the side surface of the sealing body and the auxiliary member to join the first abutment portion by friction stirring; wherein the rotary tool is moved along the second abutment portion with only the stirring pin of the rotary tool being in contact with both of the outer peripheral surface of the projecting part and the wall of the hole to join the second abutment portion by friction stirring, and wherein in the joining step, a joining condition is set in such a way that the burrs are formed on the auxiliary member. a preparation step of forming the jacket body that includes a bottom part, a peripheral wall part rising from a peripheral edge of the bottom part, a stepped bottom surface formed at a position below by one step from an end face of the peripheral wall part, a stepped side surface rising from the stepped bottom surface, and a support part erected on the bottom part and formed with a projecting part on an end face thereof, and forming a sealing body that is formed with a hole into which the projecting part is inserted and that seals an opening of the jacket body; a sealing body disposing step of disposing the sealing body on the jacket body to make the stepped side surface abut on a side surface of the sealing body to form a first abutment portion, and to make an outer peripheral surface of the projecting part abut on a wall of the hole to form a second abutment portion; an auxiliary member disposing step of disposing an auxiliary member along the first abutment portion; a joining step of moving a rotary tool along the first abutment portion and the second abutment portion to perform friction stirring; and a removing step of removing the auxiliary member on which burrs are formed, wherein the rotary tool is moved along the first abutment portion with only a stirring pin of the rotary tool being in contact with the stepped side surface, the side surface of the sealing body and the auxiliary member to join the first abutment portion by friction stirring; wherein the rotary tool is moved along the second abutment portion with only the stirring pin of the rotary tool being in contact with both of the outer peripheral surface of the projecting part and the wall of the hole to join the second abutment portion by friction stirring, and wherein in the joining step, a joining condition is set in such a way that the burrs are formed on the auxiliary member.