A resonance frequency detecting device of a multi-shaft electric motor control apparatus includes a plurality of electric motor control systems having electric motors (31, 32) for driving machines (41, 42) and controllers (21, 22) for driving the electric motors (31,32), detectors (51,52) for detecting the operating amounts of the machines (41,42), signal processors (61,62) for analyzing the frequencies of the signals of the detectors (51,52) and outputting the frequencies as resonance frequencies, and output devices (81,82) for changing the signals of the signal processors (61,62) into graphs or numeric values, a command generator (11) for giving the electric motor control system a control command for transmitting a vibration to the machines (41,42) is provided to input the signals of the detectors (51,52) to the signal processor (61) and to output them as resonance frequencies.