Since the optical energy gap (Eog) of the diamond-like carbon (DLC) varies over a wide range with the ratio of sp3 (diamond like) to sp2 (graphite like) bonds, it is expected to fabricate the high-efficiency multi-junction solar cells that consists of a single material. In the present study, we investigated the temperature dependence of the Eog of the DLC thin films by using the piezoelectric photothermal (PPT) and photoluminescence (PL) techniques. Because the PPT signal intensity of the present DLC thin film was expected to be proportional to its optical absorption coefficient, Eog was estimated by using a Tauc plot. The values of Eog of the 44%-sp3 ratio sample were estimated to 1.31 and 1.33 eV at 300 and 100 K, respectively. The temperature coefficient then became -6.5 × 10-5 eV/K, and was very smaller than that of Si. This implies that by using the DLC films for solar cell materials, the device performance is expected to be very stable by temperature change.