Femoral arterial and venous cannulation during cardiopulmonary bypass (CPB) can cause restriction of blood flow to the lower limb. To avoid complications, monitoring tissue oxygen saturation (StO₂) of the lower limb and ensuring adequate blood flow by distal perfusion (DP) are essential. A 78-year-old man underwent mitral valve replacement via a mini-right thoracotomy. During CPB, the StO₂ of the ipsilateral cannulated lower limb was decreased. Although the DP catheter was inserted into the ipsilateral femoral artery, the effect on the lower limb StO₂ was transient. Because the StO₂ of the ipsilateral lower limb improved by replacing the catheter, it was thought that the DP catheter had migrated into the deep femoral artery where collateral circulation was not developed. Among cases in which the StO₂ does not improve, even with the use of DP, it is important to confirm whether the DP is sufficiently effective.