We attempted to measure α-particles emitted by radioactive substances present in nitric acid using a single-crystal CVD diamond radiation detector, which allows for the acquisition of detailed energy information, in place of conventionally used polycrystalline diamonds and other materials, primarily designed for use in nuclear fuel reprocessing facilities. Using a 10-mm-square single-crystal CVD diamond substrate, we grew a single-crystal CVD diamond as the detector by the direct wafer method. A Ti/Au ohmic-contact anode electrode and a Pt ohmic-contact cathode electrode were formed on opposite sides of a 66-μm-thick freestanding single-crystal CVD diamond membrane by evaporation. The charge collection efficiency of the fabricated detector for 5.486 MeV α-particles was 99.1% for both holes and electrons, with energy resolutions of 0.55% and 0.54%, respectively. Instead of using the Pt cathode electrode, 5 mol/L nitric acid was used, and the α-particle spectrum from thorium oxide diffused in nitric acid was measured. The rise time of the preamplifier output signal was approximately 30 ns, and the high energy resolution enabled us to obtain a spectrum that reflected more detailed energy structures than when using conventional polycrystalline diamond detectors. Continuous measurements over 72 h confirmed the minimal influence from γ-rays.