A derivation method performed by a communication system (1a, 1b, 1c) including an access point (2a, 2b) and a wireless terminal, the access point (2a, 2b) including an accommodation station device (20a, 20b, 20c) and an extension station device (23a, 23b, 23c) connected to each other through an optical fiber transmitting an optical signal, and the wireless terminal communicating with the extension station device (23a, 23b, 23c) by using a radio-wave signal, characterized in that the derivation method comprises: a transmission step of, when a time in the accommodation station device (20a, 20b, 20c) and a time in the wireless terminal are synchronized with each other, transmitting an optical signal with a first wavelength, and an optical signal with a second wavelength; a communication start time information acquisition step of acquiring information on a first communication start time that is a time at which communication using the optical signal with the first wavelength is started, and information on a second communication start time that is a time at which communication using the optical signal with the second wavelength is started; a reception time information acquisition step of acquiring information on a first reception time that is a reception time related to a first radio-wave signal based on the optical signal with the first wavelength, and information on a second reception time that is a reception time related to a second radio-wave signal based on the optical signal with the second wavelength; and an optical fiber length derivation step of deriving a length of the optical fiber, based on the first communication start time, the first reception time, the second communication start time, the second reception time, a group velocity or a group delay time of the optical signal with the first wavelength, and a group velocity or a group delay time of the optical signal with the second wavelength. a transmission step of, when a time in the accommodation station device (20a, 20b, 20c) and a time in the wireless terminal are synchronized with each other, transmitting an optical signal with a first wavelength, and an optical signal with a second wavelength; a communication start time information acquisition step of acquiring information on a first communication start time that is a time at which communication using the optical signal with the first wavelength is started, and information on a second communication start time that is a time at which communication using the optical signal with the second wavelength is started; a reception time information acquisition step of acquiring information on a first reception time that is a reception time related to a first radio-wave signal based on the optical signal with the first wavelength, and information on a second reception time that is a reception time related to a second radio-wave signal based on the optical signal with the second wavelength; and an optical fiber length derivation step of deriving a length of the optical fiber, based on the first communication start time, the first reception time, the second communication start time, the second reception time, a group velocity or a group delay time of the optical signal with the first wavelength, and a group velocity or a group delay time of the optical signal with the second wavelength.