An exhaust gas purifier comprising: a first exhaust passage in which an exhaust gas of an engine mounted on a ship flows; a catalyst passage that is a passage branched from the first exhaust passage and selectively reduces a nitrogen oxide included in the exhaust gas; a bypass passage that is a passage branched from the first exhaust passage and bypasses the catalyst passage; a second exhaust passage that is a passage in which the catalyst passage and the bypass passage are joined at an exhaust downstream side; a casing in which at least a part of the catalyst passage and at least a part of the bypass passage are formed; an SCR catalyst that is disposed in the catalyst passage and selectively reduces a nitrogen oxide included in an exhaust gas flowing in the catalyst passage; and a backflow prevention section that prevents or reduces a backflow of an exhaust gas from the second exhaust passage to the catalyst passage, characterized in that: the backflow prevention section comprises a first backflow prevention section as the backflow prevention section; and a second backflow prevention section that is disposed between the SCR catalyst and the first backflow prevention section and prevents or reduces a backflow of an exhaust gas, wherein the casing includes an inner wall portion separating the catalyst passage and the bypass passage from each other, and a catalyst outer wall portion separating the catalyst passage and outside from each other, the second backflow prevention section is disposed to connect the inner wall portion and the catalyst outer wall portion to each other and partially has an opening, and an opening ratio at the catalyst outer wall portion side of the second backflow prevention section is smaller than an opening ratio at the inner wall portion side of the second backflow prevention section. a first exhaust passage in which an exhaust gas of an engine mounted on a ship flows; a catalyst passage that is a passage branched from the first exhaust passage and selectively reduces a nitrogen oxide included in the exhaust gas; a bypass passage that is a passage branched from the first exhaust passage and bypasses the catalyst passage; a second exhaust passage that is a passage in which the catalyst passage and the bypass passage are joined at an exhaust downstream side; a casing in which at least a part of the catalyst passage and at least a part of the bypass passage are formed; an SCR catalyst that is disposed in the catalyst passage and selectively reduces a nitrogen oxide included in an exhaust gas flowing in the catalyst passage; and a backflow prevention section that prevents or reduces a backflow of an exhaust gas from the second exhaust passage to the catalyst passage, the backflow prevention section comprises a first backflow prevention section as the backflow prevention section; and a second backflow prevention section that is disposed between the SCR catalyst and the first backflow prevention section and prevents or reduces a backflow of an exhaust gas, wherein the casing includes an inner wall portion separating the catalyst passage and the bypass passage from each other, and a catalyst outer wall portion separating the catalyst passage and outside from each other, the second backflow prevention section is disposed to connect the inner wall portion and the catalyst outer wall portion to each other and partially has an opening, and an opening ratio at the catalyst outer wall portion side of the second backflow prevention section is smaller than an opening ratio at the inner wall portion side of the second backflow prevention section.