A fuel injection control device for an internal combustion engine is provided, which comprises: a rpm sensor for detecting an engine speed; an intake pressure sensor for detecting an intake pressure; a throttle sensor for detecting the throttle opening degree; a first basic fuel injection volume calculating device for calculating a first basic fuel injection volume according to a fuel volume calculated by using the engine speed and the intake pressure as parameters; a second basic fuel injection volume calculating device for calculating a second basic fuel injection volume according to a fuel volume calculated by using the engine speed and the throttle opening degree as parameters; and a ratio calculating device for performing arithmetic operations on the first basic fuel injection volume and the second basic fuel injection volume at a desired mixture ratio, and the ratio calculating device gradually changes the mixture ratio at regular time intervals.