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Millimeter-wave imaging systems in the frequency range of 70–140 GHz have been developed for diagnostics of magnetically confined plasmas. The 70 GHz imaging system successfully measures time evolutions of both radial and axial profiles of line density and electron cyclotron emission (ECE) in a tandem mirror. The imaging system is being installed in Large Helical Device (LHD) at the National Institute for Fusion Science. In order to cover the frequency range of the second harmonic ECE on LHD, a new detector using monolithic microwave integrated circuit technology has been designed and fabricated. The detector consists of the integration of a receiving antenna, a down-converting mixer diode, and an intermediate frequency amplifier on a GaAs substrate chip. The heterodyne response up to 10 GHz was confirmed at 70–140 GHz. The optical system consisting of an ellipsoidal mirror and a flat mirror was designed by using a ray-tracing code and evaluated experimentally at 140 GHz. © 2001 American Institute of Physics.