a superimposing apparatus and method for broadcasting a 3dcg image can calculate and superimpose additional information such as characters and figures expressed by a 3dcg image on a tv frame in real time by combining a general three - dimensional graphics accelerator board and a conventional tv broadcasting hardware and devising a software processing . the superimposing apparatus performs a rendering process for rendering a 3dcg image datum by a three - dimensional graphics accelerator and a mixing process for mixing a 3dcg image and an input tv signal by a superimposer . the rendering process is carried out concurrently with the mixing process . a frame to be mixed is a previous frame advanced by one frame from a frame to be concurrently rendered . a system memory to be read out is changed to even frames and odd frames in accordance with an input tv signal so as to avoid an interference of rendering and superimposing .