The electric field in the ionosphere is measured by the double probe instrument onboard a sounding rocket. For electric field measurement, a wire antenna has been used as a sensitive sensor onboard Japanese sounding rocket. And this antenna will be used for several spacecraft in the future mission. However, its extension mechanism is complicated and it is difficult for the sounding rocket to extend a wire antenna in the ionosphere. New type antennas were developed in order to measure the electric field by the sounding rocket. They fulfill the severe requirements to the antenna system, i.e., light mass, enough stiffness, compact storage, safe extension, and reasonable test efforts. One of them, the rigid monopole antenna, was successfully used in the ISAS/JAXA S-520-23 sounding rocket experiment in September 2007.This paper describes about the basic techniques of the electric field instrument onboard the Japanese sounding rocket for measuring the electric field in the ionosphere. Then electric field data measured by the S-520-23 sounding rocket show that the new type sensor can measure the electric field in the ionosphere.