A device for parallel-plate-type atmospheric-pressure nitrogen plasma (APNP) treatment, which can generate plasma in a sealed environment with high-purity nitrogen gas, was developed. The objective of the present study was to use microstructural analyses to investigate the effect of APNP treatment at low temperatures on the fatigue properties of low-alloy steels. The APNP treatment was conducted at 473 K, which is lower than the common gas nitriding temperature. X-ray analyses revealed that a nitrogen–oxygen diffusion layer with high fatigue limit was formed on the steel by the APNP treatment at 473 K. Thus, the APNP treatment is an effective approach for increasing the fatigue limit of steels by increasing their surface hardness.