The present invention relates to an MR brake device. The MR brake device includes: a housing (2); a rotary disc (36) spaced to have a space on the inner surface of the housing (2) and installed in a spline manner to be able to be rotated together with an input shaft (3) combined with the housing (2) and detached, and including protruding parts (34) repetitively formed on the outer edge surface corresponding to the inner surface in a circumferential direction; magnetorheological fluids filling a space between the inner surface of housing (2) and the outer edge surface of the rotary disc (36); and a magnetic coil (22) installed in one part of the housing (2) to apply magnetic force to the magnetorheological fluids. A plurality of rotary discs (36) are stacked at intervals in a longitudinal direction of the input shaft (3). The protruding parts (34) have a waveform shape. The rotary discs (36) comprise a main disc (6) and a sub disc (7), which have different diameters and are stacked alternately. Therefore, the MR brake device can have high efficiency and can be easily manufactured and assembled, while providing high braking torque.