1. An evaporative emission control system including a canister for collecting fuel vapor flowing from a fuel tank, comprising: a purge control valve that controls a degree of fluid communication between the canister and an intake passage of an internal combustion engine; an air/fuel ratio deviation detecting unit that detects a deviation of an air/fuel ratio from a predetermined range under a situation where purge gas is supplied to the intake passage; a fuel injection amount calculating unit that calculates a fuel injection amount according to an expression including a correction factor for canceling an influence of the purge gas; a correction factor updating unit that updates the correction factor so as to reduce the deviation of the air/fuel ratio; an opening failure detecting unit that detects an opening failure of the purge control valve, the opening failure occurring when the purge control valve is stuck in an open state; a deviating condition determining unit that determines the presence and direction of the deviation of the air/fuel ratio when the opening failure is detected; and an initial value setting unit that assigns a rich-side initial value or a lean-side initial value to the correction factor depending upon the direction of the deviation of the air/fuel ratio when the deviating condition determining unit determines the presence of the deviation of the air/fuel ratio, wherein a difference between a reference value of the correction factor and the rich-side initial value and a difference between the reference value and the lean-side initial value are both larger than an update amount of the correction factor by which the correction factor is updated at a time by the correction factor updating unit.