We present a new concept for a phase-shifting diffraction-grating interferometer, which is intended to test concave mirrors simply using a single reflective diffraction grating and fiber optic confocal microscope focusing optics. Our configuration allows generating a high-quality reference wave from a small active grating area that can be readily fabricated with good homogeneity of grating substrate and uniformity of groove spacing. The test and reference waves encounter no other optical components, so only a small active surface area of the grating affects the test result. The fiber optic confocal microscope design adopted to solve the alignment problem between the grating and the converging wave greatly reduces systematic errors of the interferometer. In addition, the phase-shifting capability provided by translating the grating using a microactuator enables one to attain nanometer accuracy in optical figure metrology.