To obtain a flame-retardant spun yarn that increases tenacity of spun yarn while suppressing reduction in limiting oxygen index (LOI) and yet does not causes problems such as generation of poisonous gas, environmental pollution, etc.

The flame-retardant blended yarn is obtained by subjecting a modified acrylic fiber obtained by introducing a crosslinking structure to an acrylic fiber having 1-15 dtex fineness by using hydrazine to increase 0.1-10 mass% nitrogen content and then hydrolyzing the acrylic fiber having the crosslinking structure introduced therein by using an alkali metal-containing compound such as sodium carbonate, potassium carbonate, etc., and a modified cellulose fiber obtained as an alkali metal salt by introducing a carboxy group to a cellulose fiber to mixed spinning.