[en] A highly sensitive, fast responding n-InSb detector for the millimeter and submillimeter region has been fabricated and optimized for use to measure the electron cyclotron emission from tokamak plasmas, in connection with bandpass filters or a rapid scanning Fourier transform spectrometer. The detector element is made of high purity n-type InSb and is in magnetic fields up to 12 kG. The spectral responsivity, response time and noise equivalent power (NEP) of the detector have been measured. The response time of 0.2 μsec, and NEP of 1.0 x 10^-10 W/√H sub(z) at 4.2 K and 8 kG and that of 2.3 x 10^-11 W/√H sub(z) at 1.8 K and 10 kG are obtained. These values are useful for the measurements of electron cyclotron emission from tokamak plasmas. (author)