A mouthpiece (150A) has an aerosol lead-out flow passage (104), an aerosol lead-out opening (105), and exhalation discharge openings (106). The lead-out flow passage (104) includes a first flow passage (104A) extending upward, a second flow passage (104B) extending obliquely forward from the first flow passage (104A), and a corner flow passage (104C) for connecting the first flow passage (104A) and second flow passage (104B) of the lead-out flow passage (104). The mouthpiece (150A) has a curvature portion (153) formed by curving that portion of the first flow passage (104A) wall surface which crosses the center line of the first flow passage (104A). The exhalation discharge openings (106) are provided in a wall surface defining the second flow passage (104B), at a position circumferentially displaced from the wall surface portion at which the curvature portion (153) is provided. In the mouthpiece and an inhaler with the mouthpiece, although an exhalation valve and, in addition, an inhalation valve are eliminated, leakage of aerosol from a pressure regulation opening and the exhalation opening is prevented irrespective of usage manners.