Transmission start time correction method of WDM / TDM-PON system, the station-side subscriber accommodation device, the downlink signal instructing the wavelength switch to the subscriber unit, the subscriber unit transmits after wavelength switching, the subscriber unit upstream indicating a completion instruction procedure instructing including transmission start time of the uplink signal indicating the completion of wavelength switching, the subscriber unit, after completing a wavelength switching according to the instruction, the completion of wavelength switching the indicated transmission start time an instruction completion transmission step of transmitting a signal at a wavelength after the switching, the station-side subscriber accommodating apparatus measures the reception time of the uplink signal indicating the completion of wavelength switching received after wavelength switching is envisioned before wavelength switching twice as long as the time difference between the reception time of the uplink signal indicating the completion of wavelength switching it received after wavelength switching for reception time of the uplink signal indicating the completion of wavelength switching, in addition to the transmission start time of the wavelength switching preceding uplink signal Comprising a transmission start time correction procedure to set the time as a new transmission start time, a.