A mouthpiece of a brass instrument that can cause lips to vibrate efficiently while having a throat of the size allowing to produce a loud sound so that a brass instrument is made easier to blow, durability is improved, and excellent sound quality can be generated with improved sound production in high tone and low tone areas is provided.A player who blows the mouthpiece for the first time feels blowing resistance stronger than before and a reason for this feeling is that a breath more than necessary has been blown to obtain moderate blowing resistance for the player, that is, reflected pressure (pressure inside a cup) to help natural vibration of lips from the mouthpiece that has been used and, with such inefficient blowing, durability of the mouthpiece is reduced and the range of tones cannot be expanded. However, by using helpful blowing resistance (reflected pressure) causing lips to vibrate easily by a resistance part formed at least in a portion of an inner wall area of the throat to cause the blowing resistance to vibrate, lips are allowed to vibrate efficiently, a player is enabled to perform easily, and the player is made less fatigued and, resulting from these factors, high tones that could not be played can now be played, stable low tones are obtained, and the sound itself becomes deep and impressive.