The on-demand system including a plurality of terminals, a plurality of on-demand servers and a center device having a request control unit, the center device further including a power control unit for controlling power-on and power-off of said on-demand server according to the number of said terminals in operation, said power control unit including a counter which counts up in response to an on signal transmitted from said terminal which has started operation and counts down in response to an off signal transmitted from said terminal which has stopped operation and a power control part having a threshold value according to the number of said terminals in operation and the necessary number of said on-demand servers and comparing said count value and said threshold value to supply power to said on-demand server to be newly operated and stop power supply to said on-demand server whose operation is to be stopped.