Charge coupled devices (CCDs) optimized for soft X-ray detection were utilized for the solid-state imaging spectrometers (SIS) onboard the ASCA satellite, and have been in operation since its launch in 1993. We have been carefully monitoring the change in performance of SIS due to radiation damage over three years since launch. Both an increase of dark current and a decrease of charge transfer efficiency are observed, as expected from experiments in the laboratory, but their characteristics are somewhat different from those predicted before launch. This is the first detailed analysis of radiation damage to CCDs actually operated in the space environment and gives important information for the development of similar detectors for space applications.