In order to search for obscured objects at high redshifts, we have made deep far-infrared (FIR) deep survey in the Lockman Hole using ISOPHOT onboard ISO (Kawara et al. 1998, A&A, 336, L9). In this survey several tens of FIR sources have been found at 95micron and 175micron. Here we present results of optical and NIR spectroscopic followup observations performed at the W. M.Keck Observatory.