A gear structure is configured to support vertical motion of an electric pop-up monitor. More specifically, a double rack gear structure is configured to support a pop-up monitor. The rack gear structure includes an outer gear and an inner gear. The outer gear is coupled at a side of the monitor frame to operate integrally with the monitor frame, and enables the vertical motion of the monitor frame with an ascending or descending motion. The inner gear is received inside the outer gear. The inner gear performs an ascending or descending motion simultaneously with the outer gear, and supports forward and rearward or right and left shaking, which occurs in the outer gear, by maximizing a portion overlapping with the outer gear when the outer gear has ascended to the maximum.