Patent ID: 7913548

Claim:
An engine rotational speed determining device comprising: a signal inputting means for inputting a signal that indicates current supplied to an electric motor, which starts an internal combustion engine, during a starting operation of the engine by the motor; and a rotational speed determining means for determining a rotational speed of the engine in the starting operation based on a change in the current indicated by the signal input by the signal inputting means; a relaxation process performing means for performing a relaxation process for the signal input by the inputting means to eliminate the influence of electrical noise included in the signal; and a relaxation process resetting means for resetting the relaxation process at a timing when the current indicated by the signal has decreased, after an initial rapid increase thereof, to become not higher than a predetermined threshold, wherein the rotational speed determining means determines the rotational speed of the engine based on the signal that has received the relaxation process which is performed by the relaxation process performing means and reset by the relaxation process resetting means.