Disclosed herein are an apparatus for detecting a road surface state in motor-driven power steering (MDPS) and a method for controlling the apparatus. The apparatus may include a column torque sensor to detect column torque applied to a steering column and to output a column torque signal; a steering angle sensor to detect a steering angular velocity of a steering wheel; a vehicle speed sensor to detect a driving speed of a vehicle; and a road surface determiner to determine a road surface state based on a boost gain, which is calculated based on the driving speed and the column torque, and on a phase offset between the column torque and the steering angular velocity and to output a road surface determination signal.