In a micronanobubble reaction vessel 3 of wastewater treatment equipment, wastewater containing organic matter is treated with micronanobubbles. Thereafter, the wastewater is introduced into an aeration tank 7. Part of organic matter in the wastewater is oxidized in the micronanobubble reaction vessel 3 by micronanobubble treatment prior to treatment with activity of microorganisms enhanced in the aeration tank 7. After organic matter load is thus reduced, the treatment water is introduced into the aeration tank 7, in which microorganisms exist in high concentration due to submerged membranes 17, so that the organic matter in wastewater can be effectively treated. This makes it possible to accomplish miniaturization of the aeration tank 7, reduction in scale of the whole equipment and therefore reduction in initial cost. Also, a photocatalyst tank 22 is provided downstream of the aeration tank 7, so that a minute amount of organic matter unaffected by the microbial treatment alone can be oxidized at high-level by using a photocatalyst plate 24.