Our work studies the possibility of frequency doubling in InP:Fe pumped with subterahertz radiation. The attainable power of a second harmonic is calculated for an InP:Fe wafer placed in a parallel plate waveguide. It is shown that although phase matching may be compromised when the waveguide plates are introduced, there is an optimal configuration providing conversion efficiency up to 4.5% under a moderate power of 1 kW at the fundamental frequency. A zero-frequency part of nonlinear polarization is also considered in regard to diagnosis of the power at the fundamental frequency via “optical detection.”