We have developed monolithic radiation detectors based on a 0.2 mu m Fully-Depleted Silicon-on-Insulator (FD-SOI) CMOS technology. It has both a thick, high-resistivity sensor layer and a thin LSI circuit layer in a single chip. To shield the electronics part from the sensor region, we have created a buried well region under the buried oxide (BOX) layer of the SOI wafer. Two type of detectors, integration and counting types, are being developed.