An instantaneous measurement system of high-power millimeter-wave was proposed and demonstrated with a 28 GHz gyrotron at the Plasma Research Center, University of Tsukuba. The high-power detector consists of an attenuator and a linear polarized microstrip antenna with an F-class load rectifier, which is a commonly used system for radio-frequency wireless power transmission. The detector obtained the power distribution of the gyrotron output beam which showed good agreement with the infrared camera image. The rectenna array detector received 45 W RF input power with a 0.4 ms response time. The results revealed that the proposed narrow band detector is useful as an imaging sensor and power meter for high-power millimeter-wave beam output with a wide wavelength range.