An alpha-wave amplitude selector with a window slicer quickly selects and detects a time domain in which the amplitude of alpha component in brain waves exceeds or falls below a given threshold levels for use in biofeedback. A combination of a window slicer that receives alpha wave as input and an RS flip-flop connected to the window slicer significantly reduces a time-delay between a point at which the amplitude of alpha wave exceeds or falls below a preset level and a point at which a feedback stimulus is given. The window slicer outputs a high pulse when the alpha wave exceeds a high threshold slice level and a window pulse when the alpha wave enters an intermediate region between the high and a low threshold slice level, remains below the high threshold slice level and falls below the low threshold slice level at the trailing edge, whereas the RS flip-flop outputs a high-alpha phase mark based on the high pulse and a low-alpha phase mark based on the window pulse and neglects any high pulses output between the generation of the above high pulse and that of the next window pulse.