An airbag apparatus for a vehicle may include first and second inflators each configured to provide an inflation gas; a main cushion connected to the first inflator and deploying toward a passenger when the first inflator operates; a sub cushion connected to the second inflator and deploying along a first side surface of the main cushion to be overlappingly deployed with the main cushion at the first side surface and the front surface of the main cushion when the second inflator operates; and a controller configured to operate the first inflator to deploy the main cushion when a vehicle collision is detected, and configured to operate both of the first inflator and the second inflator to deploy both of the main cushion and the sub cushion when the vehicle collision is detected as an oblique collision at a least a predetermined angle.