Difference frequency generation using one 1.58-μm and two 1.06-μm distributed feedback Bragg-grating laser diodes and a ridge-type PPLN alternately provide two 3.2-μm coaxial waves resonant with individual isotopic transitions separated by 13 cm(-1). The ν(3) band R(6) A(2) allowed transition of (13)CH(4) and the ν(3) band R(6) A(2) weakly allowed transition of (12)CH(4) are an ideal pair for isotope ratio measurements. The (13)CH(4)/(12)CH(4) isotope ratio is determined for three sample gases with a relative uncertainty of 0.7 ‰, and it is confirmed that the temperature dependence is smaller than the uncertainty.