JRR-3(Japan Research Reactor No.3) is a light water moderated and cooled, heavy water reflected, pool type research reactor with maximum thermal power of 20MW. The reactor is the first domes tic research reactor in Japan, and first criticality was attained in 1962. The large scale modification of the reactor was carried out for the year 1985-1990. As the modified JRR-3 has been operated for 18 years, it is necessary to be maintained for keeping the safe and stable operation. The review of maintenance activity is constantly performed for the step-by-step update of f acilities and the improvement of the maintenance method and so on. Our investigation of the maintenance activity about the instrumentation and control device of modified JRR-3 is presented.