Abstract Observations of transient objects, such as short gamma-ray bursts and electromagnetic counterparts of gravitational wave sources, require prompt spectroscopy. To carry out prompt spectroscopy, we have developed an optical-fiber integral field unit (IFU) and connected it with an existing optical spectrograph, KOOLS. KOOLS–IFU was mounted on the Okayama Astrophysical Observatory 188 cm telescope. The fiber core and cladding diameters of the fiber bundle are 100 μm and 125 μm, respectively, and 127 fibers are hexagonally close-packed in the sleeve of the two-dimensional fiber array. We conducted test observations to measure the KOOLS–IFU performance and obtained the following conclusions: (1) the spatial sampling is ${2{^{\prime\prime}_{.}}34}$\, ± \,${0{^{\prime\prime}_{.}}05}$ per fiber, and the total field of view is ${30{^{\prime\prime}_{.}}4}$\, ± \,${0{^{\prime\prime}_{.}}65}$ with 127 fibers; (2) the observable wavelength and the spectral resolving power of the grisms of KOOLS are 4030–7310 Å and 400–600, 5020–8830 Å and 600–900, 4160–6000 Å and 1000–1200, and 6150–7930 Å and 1800–2400, respectively; and (3) the estimated limiting magnitude is 18.2–18.7 AB mag during 30 min exposure under optimal conditions.