A frequency characteristic adjusting jig attached to a loop antenna includes: a conductive first member which is located along an outer periphery of a loop for a portion of a conductor forming the loop antenna, and which is electromagnetically coupled or electrically connected to the portion of the conductor; a conductive second member which is located along the outer periphery of the loop for another portion of the conductor, and which is electromagnetically coupled or electrically connected to the other portion of the conductor; and a conductive third member which connects the first and second members together via a different path than a path formed along the loop. The first and second members are chosen to have a length along the loop such that a frequency characteristic of the loop antenna is shifted according to the length.