A brake control device of a brake system for motorbikes comprising: a front-wheel hydraulic circuit which controls a fluid pressure of a brake fluid supplied to a front-wheel-side braking part; a rear-wheel hydraulic circuit which controls a fluid pressure of a brake fluid supplied to a rear-wheel-side braking part; and an electrically-operated pump (119, 219, 300) which pressurizes the brake fluid, wherein the brake control device is capable of performing an interlocking brake control where when braking is applied to one of the front-wheel-side braking part and the rear-wheel-side braking part, braking is also applied to the other breaking part (115a, 215) in an interlocking manner with the one braking part (215, 115a), by pressurizing the brake fluid in the hydraulic circuit corresponding to the other braking part by using the electrically-operated pump, and is also capable of applying an anti-lock brake control to at least one of the front-wheel-side braking part and the rear-wheel-side braking part, characterized in that the brake control device includes a usual voltage mode where the interlocking brake control or the anti-lock brake control is performed when a voltage of electric power supplied to the brake control device is a first voltage or more, and a low voltage mode where the interlocking brake control is stopped and the anti-lock brake control is performed in a limited manner when the voltage of the electric power supplied to the brake control device is a second voltage which is lower than the first voltage or more, and an operation mode is changed over from the usual voltage mode to the low voltage mode when it is determined that the voltage of the electric power supplied to the brake control device becomes lower than the first voltage. a front-wheel hydraulic circuit which controls a fluid pressure of a brake fluid supplied to a front-wheel-side braking part; a rear-wheel hydraulic circuit which controls a fluid pressure of a brake fluid supplied to a rear-wheel-side braking part; and an electrically-operated pump (119, 219, 300) which pressurizes the brake fluid, wherein the brake control device is capable of performing an interlocking brake control where when braking is applied to one of the front-wheel-side braking part and the rear-wheel-side braking part, braking is also applied to the other breaking part (115a, 215) in an interlocking manner with the one braking part (215, 115a), by pressurizing the brake fluid in the hydraulic circuit corresponding to the other braking part by using the electrically-operated pump, and is also capable of applying an anti-lock brake control to at least one of the front-wheel-side braking part and the rear-wheel-side braking part, characterized in that the brake control device includes a usual voltage mode where the interlocking brake control or the anti-lock brake control is performed when a voltage of electric power supplied to the brake control device is a first voltage or more, and a low voltage mode where the interlocking brake control is stopped and the anti-lock brake control is performed in a limited manner when the voltage of the electric power supplied to the brake control device is a second voltage which is lower than the first voltage or more, and an operation mode is changed over from the usual voltage mode to the low voltage mode when it is determined that the voltage of the electric power supplied to the brake control device becomes lower than the first voltage.