To provide an auto-cruise control method and device preventing disturbance of a vehicle speed due to control delay even on a slope, and preventing an occupant from feeling a sense of incongruity.

A fuel injection amount to an engine corresponding to a gradient of a road detected by an image pick-up device 38 and an actual vehicle speed detected by a vehicle speed sensor 26 is stored in a vehicle speed/gradient coping fuel injection amount storage device 24 in advance. When the gradient and a gradient starting position of the road are detected by the image pick-up device 38, the corresponding fuel injection amount to the engine is read from the or calculated from the vehicle speed/gradient coping fuel injection amount storage device 24 based on a target vehicle speed and the gradient of a forward road. The target vehicle speed is maintained by a gradient combined-use control auto-cruise driving the engine by the fuel injection amount from the gradient starting position detected by the image pick-up device 38, or a position in front of the gradient starting position decided by adding a time delay till an engine rotation speed corresponding to the read or calculated fuel injection amount is reached.