In straightening a tube using a roll straightening machine provided with drum type straightening roll pairs vertically opposingly disposed, at least an outer layer portion of a roll main body is made of elastic material having spring type hardness Hs (JIS K 6301 A type) of 50 to 100 in at least three pairs of straightening rolls on an outlet side of the roll straightening machine, and an offset amount is imparted to a tube engaged in the three pairs, at least, of straightening opposing rolls such that η defined by an equation (1) ranges from 1.0×10−3 to 1.5×10−3. The offset amount is set at three positions along a tube axis where the upper and lower rolls of each pair horizontally cross. Therefore, an S/N ratio can be enhanced in an eddy current test from inside of the tube to improve inspection efficiency, and excellent quality accuracy can be ensured in a heat transfer tube used in a steam generator and the like. [ Formula ⁢ ⁢ 1 ] η = 1 R × ( d 2 ) ( 1 ) where a relationship of R=(δ2+L2)/2δ holds, assuming that d (mm) is a tube outside diameter, L (mm) is a roll stand span of the roll straightening machine, and δ (mm) is an offset amount.