The invention provides an easy and simple method of converting the resolution of image data, which is capable of generating the high-resolution image data without incongruity, does not make a circuit in the display device complicated and does not increase the power consumption. This invention can include a portable terminal device, such as a mobile telephone or a PDA, that processes and displays image data transmitted from the outside. Image data with a plurality of grayscales can be displayed by controlling the display state of each pixel in a display unit in accordance with grayscale control pulses corresponding to the number of grayscales. For example, when a 64 grayscale display is performed, a grayscale level is defined using sixty four grayscale control pulses. Thus, it is possible to emit light from pixels in the display unit by sixty four grayscale levels. Further, the resolution converting device can generate pseudo-high-resolution image data obtained by increasing the number of pixels of original image data by n multiplication and by reducing the number of grayscales of the original image data to 1/n. When displaying the pseudo-high-resolution image data, the number of grayscale control pulses is changed to 1/n by a halftone controller. That is, in the pseudo-high-resolution image data, the number of grayscales is 1/n. Therefore, the number of grayscale control pulses used for halftone display may be 1/n in accordance with the number of grayscales. Therefore, the low-resolution image data can be displayed without incongruity by converting the resolution, also, it is possible to reduce power consumption of the display unit by the reduced amount of the number of grayscale levels pulses.