A path (24) is formed in an intake tube (6), which supplies external air to a reciprocating engine (4). External air flowing from an inlet port to an outlet port of the path (24). A gaseous fuel path (26) is connected to the path (24) at an intermediate location thereof. A gaseous fuel, lighter than air, is supplied through the gaseous fuel path (26) into the path (24). A plurality of nets (8) are disposed in an area of the path (24) between the outlet port and the gaseous fuel path (26). The nets (28) are spaced along the length of the path (24) with the outer peripheral surface of each net (28) being in contact with the inner surface of the path (24).