A cooling mechanism of a combustion chamber comprising: an inner wall in contact with the combustion chamber; an outer wall; and a cooling passage arranged between the inner wall and the outer wall, wherein the cooling passage comprises: a first passage arranged along the inner wall to extend to a first direction; a second passage arranged along the inner wall to extend to the first direction; and a first connection section connected with the first passage and the second passage, wherein a longitudinal center axis of the first passage and a longitudinal center axis of the second passage are separated from each other in a second direction perpendicular to the first direction and extending along the inner wall, wherein the first passage is connected with a part of the first connection section by a first collision wall (72';72'';72''') that is arranged at an end of the first connection section in the first direction such that a part of a cooling medium which flows in operation through the first passage to the first direction and into the first connection section collides with the first collision wall (72';72'';72'''), wherein the second passage is connected with a part of the first connection section by a second collision wall (74';74'';74''') that is arranged at an end of the first connection section in the direction (-X) opposite to the first direction such that a part of the cooling medium which flows in operation through the first connection section to the direction (-X) opposite to the first direction collides with the second collision wall (74';74'';74''') and flows further in the second passage to the first direction, wherein the second collision wall (74';74";74"') has a surface approaching the inner wall as heading to the first direction, and wherein the first collision wall (72';72'';72''') has a surface departing from the inner wall as proceeding to the first direction. an inner wall in contact with the combustion chamber; an outer wall; and a cooling passage arranged between the inner wall and the outer wall, wherein the cooling passage comprises: a first passage arranged along the inner wall to extend to a first direction; a second passage arranged along the inner wall to extend to the first direction; and a first connection section connected with the first passage and the second passage, a first passage arranged along the inner wall to extend to a first direction; a second passage arranged along the inner wall to extend to the first direction; and a first connection section connected with the first passage and the second passage, wherein a longitudinal center axis of the first passage and a longitudinal center axis of the second passage are separated from each other in a second direction perpendicular to the first direction and extending along the inner wall, wherein the first passage is connected with a part of the first connection section by a first collision wall (72';72'';72''') that is arranged at an end of the first connection section in the first direction such that a part of a cooling medium which flows in operation through the first passage to the first direction and into the first connection section collides with the first collision wall (72';72'';72'''), wherein the second passage is connected with a part of the first connection section by a second collision wall (74';74'';74''') that is arranged at an end of the first connection section in the direction (-X) opposite to the first direction such that a part of the cooling medium which flows in operation through the first connection section to the direction (-X) opposite to the first direction collides with the second collision wall (74';74'';74''') and flows further in the second passage to the first direction, wherein the second collision wall (74';74";74"') has a surface approaching the inner wall as heading to the first direction, and wherein the first collision wall (72';72'';72''') has a surface departing from the inner wall as proceeding to the first direction.