Novels diffractive element in the THz waveband offers the potential to realize novel types of devices for communications, sensing, integrated optics, networks, transmission lines, and so on. To this end, diffractive planar elements fabricated on non-flat surfaces make it possible to enrich the "pool of devices" for applications including integrated optics at different waveband, including THz, and to design elements with novel properties and potentials. This can be illustrated most clearly using as an example optical element such as that for optical polychromatic computers. For instance, the diffractive element discussed above can be used as a nonlinear device for polychromatic radiation or multiplexer or a focusing element with selectivity in the multimode regime. Frequency characteristics for such elements are determined by the extent of concavity (convexity) of the surface of the element and by the direction of incidence wave onto it. Therefore, when working on a wavelength λ is not equal to λ0, the position of the focusing area in space (the amount of its displacement) and focusing properties should depend on the direction of incidence of the radiation. Hence, it is possible to distinguish between a signal incident on the "tip" of the element from that falling on its "base," simply by placing radiation receivers at the corresponding points in space. Thus, in this paper we will present detailed simulation results obtained using a parallel FDTD method and the application of the proposed device to focusing and frequency-selective properties of flat conical diffractive elements in THz waveband.