A power generation controlling method of controlling a generator connected to a four-stroke engine based on a result of measurement of an inverse of an rotation number of the four-stroke engine, characterized in that: a first step of judging whether or not a peak of the inverse of the rotation number measured in a cycle of the four-stroke engine is equal to or lower than a first threshold and equal to or higher than a second threshold, the second threshold being lower than the first threshold; a second step of, in a case where the peak is equal to or lower than the first threshold and equal to or higher than the second threshold, judging whether or not each of peaks of the inverses of the rotation numbers measured in a plurality of successive cycles following the cycle is equal to or lower than the first threshold and equal to or higher than the second threshold; a third step of, in a case where it is judged in the second step that each of the peaks is equal to or lower than the first threshold and equal to or higher than the second threshold, judging whether or not the first peak of the inverse of the rotation number measured in the first cycle following the plurality of successive cycles is equal to or lower than the second threshold; a fourth step of, in a case where the first peak is higher than the second threshold, judging whether or not the first peak is equal to or higher than a third threshold, the third threshold being higher than the first threshold; a fifth step of starting counting of an acceleration determination period after judging that the first peak is equal to or higher than the third threshold; a sixth step of judging whether or not the acceleration determination period has elapsed; a seventh step of, in a case where the acceleration determination period has not elapsed, judging whether or not the second peak of the inverse of the rotation number measured in the second cycle following the first cycle is equal to or lower than a fourth threshold, the fourth threshold being lower than the second threshold; an eighth step of, in a case where the second peak is equal to or lower than the fourth threshold, changing a target voltage of power generation by the generator a rotating shaft of which is connected to a crank shaft of the four-stroke engine from a normal state voltage to an in-acceleration voltage, the in-acceleration voltage being lower than the normal state voltage; and a ninth step of, in a case where the second peak is equal to or lower than the fourth threshold, designating the second peak as a fifth threshold. a first step of judging whether or not a peak of the inverse of the rotation number measured in a cycle of the four-stroke engine is equal to or lower than a first threshold and equal to or higher than a second threshold, the second threshold being lower than the first threshold; a second step of, in a case where the peak is equal to or lower than the first threshold and equal to or higher than the second threshold, judging whether or not each of peaks of the inverses of the rotation numbers measured in a plurality of successive cycles following the cycle is equal to or lower than the first threshold and equal to or higher than the second threshold; a third step of, in a case where it is judged in the second step that each of the peaks is equal to or lower than the first threshold and equal to or higher than the second threshold, judging whether or not the first peak of the inverse of the rotation number measured in the first cycle following the plurality of successive cycles is equal to or lower than the second threshold; a fourth step of, in a case where the first peak is higher than the second threshold, judging whether or not the first peak is equal to or higher than a third threshold, the third threshold being higher than the first threshold; a fifth step of starting counting of an acceleration determination period after judging that the first peak is equal to or higher than the third threshold; a sixth step of judging whether or not the acceleration determination period has elapsed; a seventh step of, in a case where the acceleration determination period has not elapsed, judging whether or not the second peak of the inverse of the rotation number measured in the second cycle following the first cycle is equal to or lower than a fourth threshold, the fourth threshold being lower than the second threshold; an eighth step of, in a case where the second peak is equal to or lower than the fourth threshold, changing a target voltage of power generation by the generator a rotating shaft of which is connected to a crank shaft of the four-stroke engine from a normal state voltage to an in-acceleration voltage, the in-acceleration voltage being lower than the normal state voltage; and a ninth step of, in a case where the second peak is equal to or lower than the fourth threshold, designating the second peak as a fifth threshold.