A mouthpiece of a brass instrument is provided. The mouthpiece of the brass instrument can cause lips to vibrate efficiently while having a throat of the size allowing to produce a loud sound so that the 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.A user of the mouthpiece may feel blowing resistance because a breath more than necessary has been blown to obtain moderate blowing resistance for the user. 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, lips are allowed to vibrate efficiently, the user can perform easily, and the user is made less fatigued. Also, high tones can now be played, stable low tones are obtained, and the sound itself becomes deep and impressive.