Earphone and fit adjustment method

An earphone to be worn by a user, the earphone comprising:

• a sound emitting unit that is connected to a replaceable earpiece inserted into an auricle of the user and having a plurality of sizes, and that emits a sweep signal reproduced by a terminal through the earpiece;
• a sensor that is configured to acquire, based on the sweep signal, parameters in three axial directions including an upper-lower direction, a front-rear direction, and a left-right direction of the user;
• a determination unit that is configured to determine whether a size of the earpiece is replaced or an insertion position of the earpiece in the auricle is adjusted based on an acquisition result of each of the parameters in the three axial directions;
• and
• a presentation unit that is configured to output a determination result of presence or absence of the replacement or the adjustment to the terminal.