Megawatt (MW) gyrotrons, with a wide frequency range from 14 to 300 GHz, are being developed as part of a collaborative electron cyclotron heating (ECH) study for advanced fusion devices and a demonstration power plant (DEMO). (1) Detailed designs for a 14 GHz 1 MW gyrotron are being developed for fabrication. For a 14 GHz radio frequency (RF) beam with high divergence, a calculated transmission efficiency of 94% to the corrugated waveguide coupling position was obtained initially by introducing the design concept (direct RF beam coupling by built-in waveguide) to minimize the RF transmission path. Installing a double-disk sapphire window will make it possible to develop a 1 MW gyrotron with a continuous wave (CW) at 14 GHz. (2) In experimental tests of a new 28/35 GHz dual-frequency gyrotron, the cooling characteristics of an optimal-structure double-disk sapphire window were evaluated. We confirmed that operating at 0.4 MW with a CW at 28 GHz is feasible, reaching twice the output power reported in previous studies. In a 2 ms short-pulse experimental test, maximum powers of 1.65 MW at 28.04 GHz and 1.21 MW at 34.83 GHz were achieved. (3) A design study of a 77/51 GHz dual-frequency gyrotron was performed. Oscillations above 1.5 MW for 77 GHz and 1.3 MW for 51.88 GHz are expected for a beam voltage Vk = 80 kV and beam current Ik = 60 A. (4) In an experiment with a 300 GHz gyrotron, the influence of the wave reflected from the window was reduced by tilting the output window, and mode competition in the cavity was suppressed. An output power of 0.62 MW with a pulse width of 1 ms, which is the new record for this frequency, was obtained. (5) We also performed a trial design study of a 240 GHz gyrotron for DEMO.