A FeNi ordered alloy structural body includes a support having a surface, and particles disposed on the surface of the support with gaps therebetween. Each of the particles contains an L1.sub.0-type FeNi ordered alloy phase. In a method for manufacturing the FeNi ordered alloy structural body, the support is prepared, and particles of an FeNi disordered alloy are dispersed on the surface of the support with gaps therebetween. A nitriding treatment is performed to the particles of the FeNi disordered alloy to form particles in which nitrogen is incorporated. After the nitriding treatment, a denitrification treatment is performed to desorb the nitrogen from the particles, thereby to form the particles containing the L1.sub.0-type FeNi ordered alloy phase.