Abstract: An optical sensor unit, free from electrical noise, includes a sensor case which has both lateral sides having high frequency connectors mounted thereon, and a metallic plate mounted inside the case. The metallic plate has both ends connected to inner conductors of the high frequency connectors, and its upper surface having a current and a voltage sensor unit mounted thereon for measuring the high frequency current and voltage, respectively. The current sensor unit includes total-reflection mirrors and a current sensor, and the voltage sensor unit includes further total-reflection mirrors and a voltage sensor. The voltage sensor has its upper surface having an electrode provided thereon to be connected to the sensor case. The laser light from outside is input to the current and voltage sensor units over optical fibers. The signal light output from the current and voltage sensor units is taken outside on output optical fibers.