A drying unit coupling apparatus of a liquid electrophotographic printer in which the engagement of a driving gear installed at a main frame, and a driven gear installed at a cam shaft, are prevented from disengaging when an auxiliary frame supporting the cam shaft is coupled to the main frame. The driven gear is engaged with the driving gear when the auxiliary frame and the main frame are coupled. The apparatus includes a main frame where a driving roller supports a circulating photoreceptor medium, a driving gear installed at the main frame, an auxiliary frame which is slidably installed at the main frame, a cam shaft supported at the auxiliary frame, a drying unit, a driven gear installed at the cam shaft, a cam member installed at the cam shaft, and a position fixing mechanism which fixes the position of the cam shaft when the driving gear and the driven gear are engaged, preventing the driving gear and the driven gear from deviating from each other. The drying unit includes a drying roller for absorbing a liquid carrier remaining on the photoreceptor medium, a heating roller for heating the drying roller, and a separation plate and a contact plate, all installed in a drying unit frame. The cam member selectively pushes the separation plate or the contact plate so that the drying unit frame closely contacts or separates from the main frame.