PURPOSE: To prevent hunting of a speed in an engine at its idle time, by setting battery voltage as the maximum level when a switching valve provided in a bypass passage of intake air is closed.

CONSTITUTION: When a control circuit 20 is provided in a bypass passage 7 of intake air, first, an upper limit value is set, next, a lower limit value is set. When battery voltage V in a level exceeding the lower limit value or an internal-combustion engine speed NE in a value exceeding the lower limit speed is decided "NO", the battery voltage V, if it is in a level above the upper limit value further with the internal-combustion engine speed NE in a value above the upper limit speed, is decided "YES". In this way, an amount of intake air flowing through the bypass passage is completely cut off by closing a switching valve. Then the battery voltage V is set to Vmax. In such way, hunting of a speed in an engine at its idle time can be prevented.