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