Provided is an apparatus for transitioning a millimeter wave between dielectric waveguide and transmission line using a millimeter wave transition structure formed by the dielectric waveguide, the transmission line, and a slot to transition a signal with lower losses. The apparatus includes: transmission lines disposed respectively at input and output terminals on an uppermost dielectric substrate in a signal transition direction and adapted to transition a signal; a dielectric waveguide formed by a via array disposed between top and bottom ground surfaces of a lowermost dielectric substrate in the signal transition direction as a signal transition path; and slots disposed at a signal transition path of an upper ground surface of each dielectric substrate to connect the transmission lines to the dielectric waveguide so as to transition a signal from the transmission line of the input terminal to the transmission line of the output terminal through the dielectric waveguide.