the invention the muffler 111 includes an outer box 112 comprising a pair of longitudinally separate bodies 113 113 bonded together by an adhesive 117 partly at slanted and overlapped surfaces of inner porous layers 113a the joined portions being sealed by an annular reinforcing member 115 disposed around the outer box 112 according to a twelfth embodiment shown in fig1 a muffler 121 has an outer box 122 composed of a pair of longitudinally separate tubular bodies 123 123 including respective porous layers 123a 123a having stepped portions overlapped and bonded together by an adhesive 127 the joined portion are surrounded by an annular reinforcing member 125 with the tubular separate bodies partially overlapped rhe mechanical strength and thermal insulation of the muffler 121 are increased as shown in fig1 a muffler 131 according to a thirteenth embodiment of the invention has an outer box 132 composed of two transversely separate bodies 133 133 open at one side joined together by an adhesive 137 each of the separate bodies 133 133 is made of a formed sound insulation material and composed of an inner porous layer 133a and an outer porous layer 133b there are small clearance gaps defined between confronting ends of the porous layers 133a 133a of the separate bodies 133 133 a reinforcing member 135 of a t shaped cross section is disposed within the outer box 132 and has a leg 135a inserted in each of the clearance gaps the porous layers 133a 133a and the reinforcing member.