A throttle portion is defined between an upper end of a sac chamber and a conical portion of a needle to have a throttle opening area S1. Half of an area surrounded by the throttle portion, the needle, an inner wall of the sac chamber, and a lower end extended line in a cross section of the sac chamber taken along a sac center line is an injection hole upstream area S2. A lift amount, when the throttle opening area S1 is equal to an area which is calculated by multiplying an injection hole area S3 by the number of the injection holes, is a predetermined lift amount L. A viscosity coefficient of fuel is ρ. An index value Sa, which is calculated in accordance with an equation as below, is set to 0.5 or greater. S a = ρ 2 ⁢ ∫ h = 0 h = L ⁢ ( S 1 S 2 - S 1 ) 2 ⁢ ⁢ dh