To provide a pressure reducing valve controller and a fuel injection system using the same based on difference in flow rate characteristics of each pressure reducing valve and aging of flow rate characteristics of the pressure reducing valve.

ECU 40 sets gain K according to difference between expected pressure reduction quantity Pdp(n-1) expecting that actual common rail pressure drops in previous open drive of the pressure reducing valve 24 and actual pressure reduction quantity Pr(n-1)-Pr(n) of actual pressure reduction value of fuel pressure in a common rail 20 due to previous open drive of the pressure reducing valve 24, when differential pressure ΔP(n) of actual common rail pressure Pr (n) and target common rail pressure Pt(n) is larger than predetermined value and the pressure reducing valve 24 is driven and opened in a previous time. The ECU 40 multiplies difference between previous excitation time Tprv(n-1) of the pressure reducing valve 24 and previous valve open delay time Tds(n-1) of the pressure reducing valve 24 by the gain K to calculate excitation time compensation quantity Tfb(n) for exciting the pressure reducing valve 24.

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