A muscle-activity diagnosis apparatus includes: an acquiring section acquiring a myoelectric signal from a test subject; using the myoelectric signal as an original signal, a transformed-signal generating section generating a transformed signal by performing Hilbert transformation and inverse Fourier transformation on the original signal; a phase-velocity calculation section calculating a phase velocity of the myoelectric signal on the basis of phases of the original signal and the transformed signal; and on the basis of a plurality of feature quantities of a waveform of the myoelectric signal in a unit time and the plurality of feature quantities including at least a size of amplitude of the myoelectric signal and the calculated phase velocity, a state-identifying section identifying an activity state of a muscle of a predetermined part of a body of the test subject for each of the unit time.