An exhaust gas treatment device comprising: an upstream cylinder (22, 52, 62) provided on an upstream side of an exhaust gas passage of an engine mounted on a vehicle body (2, 4); a downstream cylinder (30, 53, 63) provided on a downstream side of said upstream cylinder (22, 52, 62); a purifying part cylinder (37, 54, 64) which is provided between said upstream cylinder (22, 52, 62) and said downstream cylinder (30, 53, 63) by being connected in series with each other and accommodating a treatment member (25, 41) therein for purifying an exhaust gas; and circular gaskets provided respectively between said upstream cylinder (22, 52, 62) and said purifying part cylinder (37, 54, 64) and between said downstream cylinder (30, 53, 63) and said purifying part cylinder (37, 54, 64); wherein among connecting portions (22A, 30A, 37A, 37B) opposing with each other between said upstream cylinder (22, 52, 62) and said purifying part cylinder (37, 54, 64) and/or between said downstream cylinder (30, 53, 63) and said purifying part cylinder (37, 54, 64), the connecting portion of one cylinder is formed of a flange (23C, 31C, 36B, 38C, 23C', 38B') and the connecting portion of the other cylinder is formed of said flange (23C, 31C, 38B, 38C, 23C', 38B') and a projecting portion (44, 46, 55, 57) projecting from a position of said flange (23C, 31C, 38B, 38C, 23C', 38B') to be inserted and fitted into said one cylinder, said exhaust gas treatment device characterized in that: an engaging portion (47, 59, 71, 81), with which said gasket is engaged, is provided in said projecting portion (44, 46, 55, 57) of said other cylinder to be positioned on an outer peripheral surface of said projecting portion (44, 46, 55, 57); a support member is provided in said vehicle body (2, 4) side for supporting said upstream cylinder (22, 52, 62) and said downstream cylinder (30, 53, 63); placing legs (26, 33) are provided on said upstream cylinder (22, 52, 62) and said downstream cylinder (30, 53, 63) respectively to be mounted on said support member; a cylinder moving mechanism (27, 34) is provided between said support member and said placing leg (26, 33) to move said placing leg (26, 33) on said support member for moving said upstream cylinder (22, 52, 62) and/or said downstream cylinder (30, 53, 63) in the front and rear direction at the time of mounting and removing said purifying part cylinder (37, 54, 64); wherein said projecting portion (44, 46, 55, 57) of said other cylinder is inserted and fitted to or is disengaged from said one cylinder by said cylinder moving mechanism (27, 34). an upstream cylinder (22, 52, 62) provided on an upstream side of an exhaust gas passage of an engine mounted on a vehicle body (2, 4); a downstream cylinder (30, 53, 63) provided on a downstream side of said upstream cylinder (22, 52, 62); a purifying part cylinder (37, 54, 64) which is provided between said upstream cylinder (22, 52, 62) and said downstream cylinder (30, 53, 63) by being connected in series with each other and accommodating a treatment member (25, 41) therein for purifying an exhaust gas; and circular gaskets provided respectively between said upstream cylinder (22, 52, 62) and said purifying part cylinder (37, 54, 64) and between said downstream cylinder (30, 53, 63) and said purifying part cylinder (37, 54, 64); an engaging portion (47, 59, 71, 81), with which said gasket is engaged, is provided in said projecting portion (44, 46, 55, 57) of said other cylinder to be positioned on an outer peripheral surface of said projecting portion (44, 46, 55, 57); a support member is provided in said vehicle body (2, 4) side for supporting said upstream cylinder (22, 52, 62) and said downstream cylinder (30, 53, 63); placing legs (26, 33) are provided on said upstream cylinder (22, 52, 62) and said downstream cylinder (30, 53, 63) respectively to be mounted on said support member;