A high-resolution soft X-ray emission spectrometer has been developed for site-specific electronic structure analysis of functional materials in the photon energy range of 50–600 eV on the in-vacuum undulator beamline BL3U of the UVSOR facility. In order to realize the high detection efficiency without sacrificing the energy resolution to the signal intensity, the present X-ray emission spectrometer adopts an optical design, which was originally adopted in the field of astrophysics such as Chandra X-ray observatory. This new-type X-ray emission spectrometer consists of a Wolter type I mirror, a freestanding transmission grating, and a charge coupled device (CCD) detector, allowing the focusing with 20 times larger acceptance angle (1.5 × 10−3 sr) than commercially available compact spectrometers with reflection-type gratings such as VG-SCIENTA XES350. The X-ray emission energy resolution of 15–34 meV has successfully been achieved in the photon energy of 60–100 eV by using a transmission grating with the groove density of 5555 lines/mm.Highlights► We develop a highly efficient and high-resolution soft X-ray emission spectrometer. ► We combine a transmission grating, a Wolter type I mirror, and normal-incident CCD. ► X-ray emission resolution proves to be 15–34 meV in the photon energy of 60–100 eV.