Disclosed herein are an apparatus and method for cultivating microalgae using effluent from sludge treatment. The apparatus includes an advanced sewage treatment apparatus, a sludge treatment apparatus and a microalgae cultivation apparatus, the sludge treatment apparatus including: a first aerobic reactor which is operated under aerobic conditions and serves to ferment sludge; a second aerobic reactor which is operated in a state in which air is injected in an amount larger than that in the first aerobic reactor, and serves to increase the fermentation activity of microorganisms in the sludge and degrade the sludge; a membrane bio-reactor (MBR) which serves to receive effluent from the second aerobic reactor and biologically remove high-concentration organic matter from the effluent by the action of aerobic microorganisms while removing total suspended solids using a membrane, wherein the effluent discharged from the MBR reactor is supplied to the microalgae cultivation apparatus.