An electric field sensor includes a probe, a cylindrical probe guide, an insulating member, a preload spring and a connector. The probe serves as an inner conductor of a coaxial transmission path and has a portion forming a monopole antenna at a tip end to be in constant contact with a microwave transmission window by a pressing force of a built-in spring thereof. The probe guide is disposed at an outer side of the probe and serves as an outer conductor of the coaxial transmission path. The insulating member is disposed between the probe and the probe guide. The preload spring preloads the probe guide downward and presses the probe guide so that the tip end of the probe guide comes in constant contact with the planar slot antenna. The connector is connected to the probe and the probe guide to connect coaxial signal cables for extracting signals.