Chemical resistance to high concentrations of sodium hydroxide solutions on diamond-like carbon (DLC) films was investigated by immersion testing. DLC films were deposited on silicon (Si) and polytetrafluorethlene (PTFE) plates. These sample were put into a 17 at.% sodium hydroxide solution. The DLC films on PTFE were maintained for 48 h, although most of the DLC films deposited on Si were removed. To understand this reaction, the results of immersion tests using 10 wt.% sodium hydroxide solution were observed in situ. Many bubbles appeared after 14 h. The number of the bubbles, which were reaction points, increased with the duration of the immersion test. We observed areas where the DLC film remained, and these places experienced no change. DLC films were peeled away because of the corrosion of the Si substrate by the sodium hydroxide. These bubbles were probably hydrogen gas, which results from the reaction between silicon and sodium hydroxide. Hence, the DLC films contained weak points against sodium hydroxide solution, or there were extremely small through holes in the DLC film.