A sound output device configured to be installed in a vehicle, the sound output device comprising: a driving-status detector for detecting a driving status of the vehicle; a sound signal generator for generating a reference signal in response to the detected driving status; a first unit for processing the generated reference signal to generate a first reference signal; a second unit for processing the generated reference signal to generate a second reference signal; a first sound output part for outputting the first reference signal; a second sound output part for outputting the second reference signal; a phase shifter for shifting a phase of the reference signal such that the respective reference signals output from the first sound output part and the second sound output part have a phase difference between the respective reference signals, wherein the sound signal generator stores a data table containing a plurality of sampling values at a plurality of points within 1/4 of a cycle of a waveform of the reference signal, wherein the first unit generates the first reference signal by extracting the plurality of sampling values at output sampling periods from the data table at intervals in response to the detected driving status, wherein the second unit generates the second reference signal by extracting the plurality of sampling values at the output sampling periods from the data table at the intervals, and wherein the phase shifter sets a point out of the plurality of points which is to be extracted by the first unit such that the point is apart from a point out of the plurality of points to be extracted by the second unit by a number obtained by accumulating a predetermined increment at the output sampling periods to be added to a current number toward a target number corresponding to a phase for a new driving status in order to gradually increase the amount of phase to be shifted. a driving-status detector for detecting a driving status of the vehicle; a sound signal generator for generating a reference signal in response to the detected driving status; a first unit for processing the generated reference signal to generate a first reference signal; a second unit for processing the generated reference signal to generate a second reference signal; a first sound output part for outputting the first reference signal; a second sound output part for outputting the second reference signal; a phase shifter for shifting a phase of the reference signal such that the respective reference signals output from the first sound output part and the second sound output part have a phase difference between the respective reference signals, wherein the sound signal generator stores a data table containing a plurality of sampling values at a plurality of points within 1/4 of a cycle of a waveform of the reference signal, wherein the first unit generates the first reference signal by extracting the plurality of sampling values at output sampling periods from the data table at intervals in response to the detected driving status, wherein the second unit generates the second reference signal by extracting the plurality of sampling values at the output sampling periods from the data table at the intervals, and wherein the phase shifter sets a point out of the plurality of points which is to be extracted by the first unit such that the point is apart from a point out of the plurality of points to be extracted by the second unit by a number obtained by accumulating a predetermined increment at the output sampling periods to be added to a current number toward a target number corresponding to a phase for a new driving status in order to gradually increase the amount of phase to be shifted.