A holographic display apparatus (100; 200) comprising: a light source configured to emit an illumination light; a spatial light modulator comprising a plurality of display pixels (130a, 130b) two-dimensionally arranged at a first resolution on a regular pixel lattice to display a hologram pattern for modulating the illumination light to reproduce a holographic image; a mask member (140; 140'; 240) disposed to face the spatial light modulator and comprising an array of a plurality of light transmission patterns irregularly arranged at a second resolution higher than the first resolution of the regular pixel lattice; and an image processor configured to: generate a hologram data array comprising information of the holographic image to be reproduced at the first resolution or a resolution less than the first resolution; perform an off-axis phase computation on the hologram data array at the second resolution, the off-axis phase computation comprising multiplying the hologram data array by a predetermined phase array in order to adjust a position where the holographic image is reproduced; generate computer generated hologram data at the first resolution from the hologram data array multiplied by the predetermined phase array at the second resolution; and provide the CGH data to the spatial modulator to display the hologram pattern. a light source configured to emit an illumination light; a spatial light modulator comprising a plurality of display pixels (130a, 130b) two-dimensionally arranged at a first resolution on a regular pixel lattice to display a hologram pattern for modulating the illumination light to reproduce a holographic image; a mask member (140; 140'; 240) disposed to face the spatial light modulator and comprising an array of a plurality of light transmission patterns irregularly arranged at a second resolution higher than the first resolution of the regular pixel lattice; and an image processor configured to: generate a hologram data array comprising information of the holographic image to be reproduced at the first resolution or a resolution less than the first resolution; perform an off-axis phase computation on the hologram data array at the second resolution, the off-axis phase computation comprising multiplying the hologram data array by a predetermined phase array in order to adjust a position where the holographic image is reproduced; generate computer generated hologram data at the first resolution from the hologram data array multiplied by the predetermined phase array at the second resolution; and provide the CGH data to the spatial modulator to display the hologram pattern. generate a hologram data array comprising information of the holographic image to be reproduced at the first resolution or a resolution less than the first resolution; perform an off-axis phase computation on the hologram data array at the second resolution, the off-axis phase computation comprising multiplying the hologram data array by a predetermined phase array in order to adjust a position where the holographic image is reproduced; generate computer generated hologram data at the first resolution from the hologram data array multiplied by the predetermined phase array at the second resolution; and provide the CGH data to the spatial modulator to display the hologram pattern.