A Fe--Ni based permalloy comprises Ni: 30 85 wt %, C: not more than 0.015 wt %, Si: not more than 1.0 wt %, Mn: not more than 1.0 wt %, P: not more than 0.01 wt %, S: not more than 0.005 wt %, O: not more than 0.0060 wt %, Al: not more than 0.02 wt % and, if necessary, not more than 15 wt % of at least one selected from the group consisting of Mo, Cu, Co and Nb within a range of not more than 20 wt % in total.