An internal combustion engine defining a combustion chamber configured to generate a swirl flow and comprising: a fuel injection valve that injects fuel to the combustion chamber, a plurality of injection holes being provided in the fuel injection valve; and a piston including a cavity exposed to the combustion chamber, wherein the cavity includes a bottom wall surface having a shape which changes its height in the direction of the swirl flow, characterised in that for two adjacent injection holes respectively injecting the fuel to adjacent regions of the combustion chamber in which angular velocities of the swirl flow are relatively lower and then higher in a rotational direction of the swirl flow, an injection hole interval between the two adjacent injection holes is set to be smaller than a reference interval, the region in which the angular velocity of the swirl flow is lower is adjacent to a first portion of the bottom wall surface, the region in which the angular velocity of the swirl flow is higher is adjacent to a second portion of the bottom wall surface, and a gradient of the bottom wall surface changing, in the direction of the swirl flow, is greater in the first portion than in the second portion, and for two adjacent injection holes respectively injecting the fuel to adjacent regions of the combustion chamber in which angular velocities of the swirl flow are relatively higher and then lower in the rotational direction of the swirl flow, an injection hole interval between the two adjacent injection holes is set to be greater than the reference interval, the region in which the angular velocity of the swirl flow is higher is adjacent to the second portion of the bottom wall surface, the region in which the angular velocity is lower is adjacent to an intermediate portion of the bottom wall surface, and a gradient of the bottom wall surface changing, in the direction of the swirl flow, is smaller in the second portion than in the intermediate portion. a fuel injection valve that injects fuel to the combustion chamber, a plurality of injection holes being provided in the fuel injection valve; and a piston including a cavity exposed to the combustion chamber, wherein the cavity includes a bottom wall surface having a shape which changes its height in the direction of the swirl flow, characterised in that for two adjacent injection holes respectively injecting the fuel to adjacent regions of the combustion chamber in which angular velocities of the swirl flow are relatively lower and then higher in a rotational direction of the swirl flow, an injection hole interval between the two adjacent injection holes is set to be smaller than a reference interval, the region in which the angular velocity of the swirl flow is lower is adjacent to a first portion of the bottom wall surface, the region in which the angular velocity of the swirl flow is higher is adjacent to a second portion of the bottom wall surface, and a gradient of the bottom wall surface changing, in the direction of the swirl flow, is greater in the first portion than in the second portion, and for two adjacent injection holes respectively injecting the fuel to adjacent regions of the combustion chamber in which angular velocities of the swirl flow are relatively higher and then lower in the rotational direction of the swirl flow, an injection hole interval between the two adjacent injection holes is set to be greater than the reference interval, the region in which the angular velocity of the swirl flow is higher is adjacent to the second portion of the bottom wall surface, the region in which the angular velocity is lower is adjacent to an intermediate portion of the bottom wall surface, and a gradient of the bottom wall surface changing, in the direction of the swirl flow, is smaller in the second portion than in the intermediate portion.