We have developed a design method for a photonic crystal directional coupler switch (PCDCSw) with a short switching length and wide bandwidth. Usually, the relationship between the switching length and bandwidth has a trade-off. To overcome this trade-off relation, we invented the suitable dispersion curve of eigenmodes of the PCDCSw. To actually obtain this dispersion curve, we examine the electromagnetic field of an ordinary PCDC at each wavenumber, and modify the even-mode mainly by enlarging the radius of the airs-holes and by shifting the position of the air-holes closer to the waveguides in the photonic crystals. We confirm using numerical simulation that the switching length of the new designed structure is only 4% of that for the ordinary PCDC.