Silicon-on-insulator (SOI) technology is being investigated for monolithic pixel device fabrication. The SOI wafers by UNIBOND allow the silicon resistivity to be optimized separately for the electronics and detector parts. We have fabricated pixel detectors using fully depleted SOI (FD-SOI) technology provided by OKI Semiconductor Co. Ltd. The first pixel devices consisting of 32times32 matrix with 20 mum times 20 mum pixels were irradiated with ⁶⁰Co gamma's up to 0.60 MGy and with 70-MeV protons up to 9.3times10 ⁶⁰Co p/cm². The performance characterization was made on the electronics part and as a photon detector from the response to reset signals and to laser. The electronics operation was affected by radiation-induced charge accumulation in the oxide layers. Detailed evaluation of the characteristics changes in the transistors was separately carried out using transistor test structures to which a wider range of irradiation, from 0.12 kGy to 5.1 MGy, was made with ⁶⁰Co gamma's.