Abstract: A brass instrument such as a trombone has a main tube member formed with a main air passage, a plurality of bypassing tube members respectively formed with bypassing air passages, and a plurality of valve units selectively coupling the main air passage to the bypassing air passages for variation in pitch of the sound produced therein, and each of the valve units is provided with two three port valve assemblies linked with each other for simultaneously being shifted between two positions, in which each of the valve assemblies has three ports angularly paced apart from one another by angles of about 120 degrees, so that the valve assemblies are less causative of resistance to the vibratile air column, thereby providing a comfortable feeling to the player.