PROBLEM TO BE SOLVED: To achieve an input interface of a line of sight with a simple configuration.SOLUTION: The earphone 10 is an earphone 10 mounted to a user, and is mounted to the external auditory canal of each of right and left ears of a user. The earphone 10 includes: two ear chips 12a and 12b made of conductive rubber containing silver or silver chloride an earphone holding tool 13a that is brought into contact with the user and inputs a reference potential and two output units 14a and 14b for outputting, with respect to the reference potential, potentials input from respective ear chips 12a and 12b.