Active aluminum (Al) (Al present in Al-humus complexes and allophanic clays) possesses important characteristics for the accumulation of soil organic matter (SOM) in Andosols. Considering the function of the active Al, we studied the effect of an amorphous Al hydroxide gel (Al gel) treatment on the decomposition of SOM in the Andosols. By adding the Al gel to four A horizon samples, we prepared 4 levels of the Al additions (0 to 1% Al) of Al gel-treated soils. We determined the soil respiration (CO2 release) rates and the fluorescein diacetate (FDA) hydrolysis activity over a 21-day period. In all the soil samples, both the respiration rates and FDA hydrolysis activity were proportionally reduced with the increasing Al gel concentration. The patterns of the temporal change in the respiration rates and FDA hydrolysis activity were different; the respiration rate was higher during the initial period (days 0 – about 7), then gradually decreased. In contrast, the FDA hydrolysis activity gradually increased over 0 – 7 days and remained at an almost constant level thereafter. Based on these results, the respiration rates and the FDA hydrolysis activity are considered to represent the availability of organic carbon (OC) at that time and the stable microbial activity, respectively. Because the water soluble organic carbon (OC) decreased with the Al gel treatment, the formation of Al-organic matter complexes possibly occurred. The decline in the FDA hydrolysis activity with the Al gel treatment indicated a physical effect such as the coating or embedding of microorganisms by the formation of soil aggregates. This was suggested by the fact that the FDA hydrolysis activity recovered after an ultrasonic treatment. This study confirmed the inhibiting effect of the Al gel treatment on the SOM decomposition in the Andosol samples, and its factors were considered to be the increase in the persistency of the SOM and suppression of the microbial activity.