A controller of a brake system for a motorcycle, the brake system for a motorcycle including: a first brake operation section that is provided on a handlebar of the motorcycle; a front-wheel brake mechanism that brakes a front wheel of the motorcycle in an interlocking manner with at least the first brake operation section; a second brake operation section that is provided on a trunk of the motorcycle; and a rear-wheel brake mechanism that brakes a rear wheel of the motorcycle in an interlocking manner with at least the second brake operation section, the controller comprising: a brake action control section that at least governs an action of the rear wheel brake mechanism, a brake operation determination section that determines presence or absence of a user's operation of the first brake operation section; and a slip-down determination section that determines whether the motorcycle is in a slip-down state where the motorcycle travels backward while the front wheel is braked, wherein in the cases where the brake operation determination section determines that the user's operation of the first brake operation section is present and the slip-down determination section determines that the motorcycle is in the slip-down state when a road surface is an upward slope, the brake action control section executes an emergency control action that causes the rear-wheel brake mechanism to generate a braking force that brakes the rear wheel. a first brake operation section that is provided on a handlebar of the motorcycle; a front-wheel brake mechanism that brakes a front wheel of the motorcycle in an interlocking manner with at least the first brake operation section; a second brake operation section that is provided on a trunk of the motorcycle; and a rear-wheel brake mechanism that brakes a rear wheel of the motorcycle in an interlocking manner with at least the second brake operation section, the controller comprising: a brake action control section that at least governs an action of the rear wheel brake mechanism, a brake operation determination section that determines presence or absence of a user's operation of the first brake operation section; and a slip-down determination section that determines whether the motorcycle is in a slip-down state where the motorcycle travels backward while the front wheel is braked, wherein in the cases where the brake operation determination section determines that the user's operation of the first brake operation section is present and the slip-down determination section determines that the motorcycle is in the slip-down state when a road surface is an upward slope, the brake action control section executes an emergency control action that causes the rear-wheel brake mechanism to generate a braking force that brakes the rear wheel. a brake action control section that at least governs an action of the rear wheel brake mechanism, a brake operation determination section that determines presence or absence of a user's operation of the first brake operation section; and a slip-down determination section that determines whether the motorcycle is in a slip-down state where the motorcycle travels backward while the front wheel is braked, wherein in the cases where the brake operation determination section determines that the user's operation of the first brake operation section is present and the slip-down determination section determines that the motorcycle is in the slip-down state when a road surface is an upward slope, the brake action control section executes an emergency control action that causes the rear-wheel brake mechanism to generate a braking force that brakes the rear wheel.