We developed an effective microreactor for photoreactions by using a black aluminum oxide channel substrate with low reflectance and good heat conductance. The performance of this microreactor was tested by carrying out a photoreactive synthesis of benzopinacol and acetone from benzophenone and 2-propanol. In the microreactor made of quartz glass, the channels were blocked at smaller flow rates, because of precipitation from the product solution owing to the vaporized liquid components such as 2-propanol and acetone. However, in the microreactor that used the black aluminum oxide channel substrate, the channels remained unblocked and clear at the identical flow rates. The yield reached 36% with an irradiation time of 469.2 s and was improved by more than 30% when compared to the values obtained by the batch method. Furthermore, the temperature of the light emitting diode (LED) source remained at about 303 K with the microreactor that used the black aluminum oxide channel substrate with low reflectance, while the temperature of the LED source increased to 313-323 K when the microreactors made of quartz glass were used. Therefore, the microreactor that used the black aluminum oxide channel substrate was found to be effective for improving the yields of photoreactions.