Abstract: A method of manufacturing hollow steel bars comprising the steps of preparing a hollow billet with the dimensions meeting a condition expressed by the following formula (1) by piercing a steel billet with a piercer after heating, inserting a mandrel as an inner surface sizing tool into a hollow billet, and then rolling the hollow billet on a cross-rolling mill having three rolls arranged around a pass line to provide plastic working for reduction of the outside diameter and adjustment of the wall thickness of the hollow billet so as to meet a condition expressed by the following formula (2), and a manufacturing apparatus comprising an electric resistance heating unit, the piercer, and the cross-rolling mill, whereint.sub.0 /d.sub.0 >0.1 (1)Rt<0.55Rd (2)wheret.sub.0 =wall thickness of hollow billet before cross rollingd.sub.0 =outside diameter of hollow billet before cross rollingRt=wall thickness reduction (%), Rt=(t.sub.0 -t.sub.1)/t0.times.100Rd=outside diameter reduction (%), Rd=(d.sub.0 -d.sub.1)/d.