Solubility of MnS in Fe-Ni alloys has been investigated by "in-situ" observation with a confocal scanning laser microscope of specimens with various Mn, S and Ni contents which were heated and cooled in an infrared image furnace. Precipitation behavior of MnS at the surface of the specimens on cooling was monitored on a CRT and was recorded on videotape. Most MnS precipitates are found to form on alumina inclusion particles and grow in triangular (pyramidal) or rod-like shape. From precipitation temperatures obtained by the "in-situ" observation, the equilibrium constant of the reaction MnS_←^→Mn+S is determined as logK=log[%Mn][%S] f _S^Mn= –9420/T+3.67, at T=1329–1493K for Fe–42mass%Ni alloy. The K value for γ-Fe at 1438K is in good agreement with that by Turkdogan et al., and Ni is found to increase the solubility of MnS.