PURPOSE: To obtain a reactor for a semiconductor circuit which can eliminate a current spike or ringing which causes noise generation by combining a core composed of amorphous magnetic alloy and a conductor.

CONSTITUTION: Amorphous magnetic alloy, introduced as a core material, has such characteristics that B1 (flux density under a magnetic field of one oersted) at 100kHz is more than 6KG and coercive force (Hc) is less than 0.5 oersted and square ratio (Br/B1, Br: residual flux density) is more than 0.8, and is expressed as Co(a)M(b)M'(c)Y(d) (where M means one element selected from Fe and Mn, and M' means one or two elements selected from transition metals except Fe and Mn, Y means one or two elements selected from Si, B, P and C and a+b+ c+d=100, 0≤b≤10, 0≤c≤10 and 50≤a≤80) and preferably M' may be selected from Ca, Ni, Nb, Mo, W, Zr, Ti, V, Ta, Hf, Re, Cu and Y.