We developed a cavity ringdown spectrometer by utilizing a step-scanning and dithering method for matching laser wavelengths to optical resonances of an optical cavity. Our approach is capable of working with two and more lasers for quasi-simultaneous measurements of multiple gas species. The developed system was tested with two lasers operating around 1654 nm and 1658 nm for spectral detections of ¹²CH₄ and its isotope ¹³CH₄ in air, respectively. The ringdown time of the empty cavity was about 340 µs. The achieved high detection sensitivity of a noise-equivalent absorption coefficient was 2.8 × 10⁻¹¹ cm⁻¹ Hz^−1/2 or 1 × 10⁻¹¹ cm⁻¹ by averaging for 30 s. The uncertainty of the high precision determination of ${\mathsf{\delta }}^{13}{\mathrm{CH}}_4$ in air is about 1.3‰. Such a system will be useful for future applications such as environmental monitoring.