Although nanoparticles capable of providing an extremely large active surface area have highly marked advantages, when a PEFC electrode utilizing nanoparticles is used for a prolonged period of time, the catalyst nanoparticles on carrier of the PEFC electrode because of the nano-size thereof migrate and aggregate together to result in a rapid loss of activity. Thus, there is a demand for inhibition of the above aggregation so as to prevent any drop of catalytic activity. According to the present invention the aggregation of nanoparticles can be inhibited by catalyst nanoparticles containing Pt wherein a porous matter containing an inorganic oxide is disposed on the surface of the catalyst nanoparticles. When use is made of nanoparticles whose surface has undergone specific modification, excellent activity can be realized. Therefore, there are provided surface-modified nanoparticles and catalyst and further a PEFC electrode utilizing these nanoparticles.