a data signal si determined as valid data is outputted to a data buffer 3 while outputting a ps power supply control start signal s 3 to a ps controller 4 in response to the ps control start signal s 3 the ps controller 4 outputs a ps control signal s 5 to the power supply portion 5 at a timing before the data signal s 2 is outputted from the data buffer 3 the ps control signal s 5 is a signal for suppressing a variation of a power supply voltage pv supplied from the power supply portion 5 for the signal processor 6 to perform the signal processing such as serving to increase the switching frequency or output voltage of the power supply portion 5 the signal processor 6 generates a processing completion signal s 4 at the time of process completion to be provided to the ps controller 4 this ps controller 4 provides the ps control signal s 5 to the power supply portion 5 to restore the control state switching frequency or output voltage to the original state thus the data signal s 2 is sent to the signal processor 6 to predict in advance a reduction of the ps voltage pv of the power supply portion 5 due to an increase of the consumption power in the signal processor 6 thereby enabling the power supply portion 5 to provide a power supply without decreasing the ps voltage pv of the power supply portion 5 fig2 depicts an embodiment 1 of a power supply control method and device according to the present invention this embodiment is applied to the optical interface board ethernet.