PURPOSE: To prevent any increase in brake stepping force of a brake servo device by increasing a load which is formed in an intake port when brake operates in the low load operation condition including idling time of an internal combustion engine.

CONSTITUTION: A brake servo means F introduces negative pressure which is formed in an intake port by means of a connection pipe to be connected to the intake port positioned downstream from a shut-off valve B and amplifies brake force when the brake operates. A negative pressure increase means G increases negative pressure which is formed in an intake port in such a case that operation of a brake is detected by a brake operation detection means E when the idling detection means A detects that the low load operation condition including idling time is prevent. Consequently, it is possible to prevent increase of brake stepping force by using the brake servo means F effectively.