An ignition system for internal combustion engines is constructed so that, in response to each ignition signal from ignition signal generating means, through a predetermined angle before a succeeding ignition timing, a storage quantity of storing means is gradually increased from an initial value and then it is gradually decreased, and a time point when the storage quantity of the storing means has been decreased to reach the initial value is detected by first storage quantity detecting means to thereby turn on a power transistor for turning on and off a primary current of an ignition coil at that time point, and further the fact that the storage quantity of the storing means has been decreased to reach a set value larger than the initial value is detected by second storage quantity detecting means to thereby change a decrease rate of the storage quantity of the storing means.