PURPOSE: A computer image signal scan conversion and synthesis apparatus using dual port memory is provided to solve an error of interchangeability to be occurred when VGA and letter generator is installed with additional card method by using a dual port memory to synthesize letter and graphic in real time at an image of NTSC or PAL type supplied from outside. CONSTITUTION: An analog to digital converter(2) converts and outputs an image signal of analog that is input through an image input part(1) into digital. An NTSC encoder(3) makes an encoding a digital R, G, B color data signal by NTSC type. A buffer(4) makes a buffering the image signal that is made an encoding by the NTSC encoder(3). A system control part(10) controls each of system part to convert the digital R, G, B color data signal which is made from VGA according to a timing signal of VGA and NTSC into a digital image signal scanned with NTSC type. A dual port memory(20) stores the digital R, G, B color data signal that is of VGA data hitting a timing of VGA signal according to the motion of the system control part(10) and then transmits the stored VGA data according to the orders of screen arrangement. A FIFO(first in first out) part(30) outputs the VGA data which is transmitted from the dual port memory(20) to an outside input NTSC type image signal by the NTSC encoder(2) during a regular designated time. A key signal generator(40) generates a key signal to synthesize VGA graphic and letter accurately to the outside input NTSC type image signal. A superimpose part(50) synthesizes a computer graphic and letter image signal which is made a buffering to the NTSC image signal input from the image input part(1) according to the key signal which is generated from the key signal generator(40).