Exhaust gas purifier for internal combustion engine and method for purifying exhaust gas

A reversal device (71) reverses upstream and downstream sides of a particulate filter (70). First and second collecting surfaces of a collecting wall (54) are used alternately to collect particulates. In order to ensure that a portion of exhaust gas bypasses the particulate filter (70) while a valve body (71a) is switched from one position to the other position to reverse the upstream and downstream sides of the particulate filter (70), the reversal device (71) switches the valve body (71a) as soon as the amount of particulates discharged from a combustion chamber of an engine becomes equal to or smaller than a set discharge amount.