A derivation method performed by a communication system including an access point and a wireless terminal, the access point including an accommodation station device and an extension station device connected to each other through an optical fiber transmitting an optical signal, and the wireless terminal communicating with the extension station device by using a radio-wave signal, characterized in that the derivation method comprising the following steps by the accommodation station device: a transmission step of transmitting, to the extension station device, 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 in the accommodation station device, 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 in the accommodation station device; a reception time information acquisition step of acquiring information on a first reception time that is a reception time related to the optical signal with the first wavelength in the accommodation station device, and information on a second reception time that is a reception time related to the optical signal with the second wavelength in the accommodation station device; a transmission time period derivation step of deriving a first round trip time that is a time period from the first communication start time until the first reception time, and deriving a second round trip time that is a time period from the second communication start time until the second reception time, wherein the round trip times are derived for the signals between the accommodation station device and the wireless terminal; and an optical fiber length derivation step of deriving a length of the optical fiber, based on the first round trip time, the second round trip 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 transmitting, to the extension station device, 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 in the accommodation station device, 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 in the accommodation station device; a reception time information acquisition step of acquiring information on a first reception time that is a reception time related to the optical signal with the first wavelength in the accommodation station device, and information on a second reception time that is a reception time related to the optical signal with the second wavelength in the accommodation station device; a transmission time period derivation step of deriving a first round trip time that is a time period from the first communication start time until the first reception time, and deriving a second round trip time that is a time period from the second communication start time until the second reception time, wherein the round trip times are derived for the signals between the accommodation station device and the wireless terminal; and an optical fiber length derivation step of deriving a length of the optical fiber, based on the first round trip time, the second round trip 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.