A fillet weld joint of steel formed by arc welding, wherein a martensitic transformation starting temperature (Ms point) of a weld metal is 400° C. or above and 550° C. or below, a value obtained by dividing a toe radius ρ of a toe part of welding by a sheet thickness t of a base metal (ρ/t) is 0.25 or above, an expression Ms(° C.)≦375×[ρ/t]+320 is satisfied, and a crack defect does not exist in the fillet weld joint.