We are developing a technique for the investigation of archaeological ruins using cosmic ray imaging (muography), a nondestructive imaging technique that utilizes muons in cosmic ray air showers. We use nuclear emulsion plates as muon detectors. The features of being electric power supply-free, lightweight, and compact are highly advantageous for installing the detector in narrow structures inside archaeological ruins such as pyramids and catacombs for the observation of cosmic ray muons. In 2016 and 2017, in ScanPyramids, cosmic ray imaging with nuclear emulsion plates revealed two unknown voids named SP-NFC and SP-BV inside the Khufu’s Pyramid in Egypt. To analyze the more detailed geometry of these voids, we have installed multiple nuclear emulsion plates inside the Pyramid of Khufu in 2019. Through these observations, we successfully revealed the position and shape of SP-NFC with high precision, leading to the direct imaging of the space using a fiberscope. This achievement not only represents the first instance of directly confirming a void discovered through cosmic ray imaging but also has a significant impact on archaeology. Furthermore, in 2022, we started to search for the hidden inner structure of the Pyramid of Khafre’s Pyramid. Our technology has been used not only for pyramids in Egypt, but also in the investigation of the temple pyramid at the Copan ruins in Honduras of the ancient Mayan civilization, and the underground ruins in Naples, Italy. Cosmic ray imaging is expected to become a new method for archaeological site surveys in the future.