To provide a dual curing type composite resin for core build-up having improved curing property by chemical polymerization with improved mechanical strength of a cured body under the condition of no light irradiation and with a shortened setting time, the composite resin is composed of a first component including a (meth)acrylate polymerizable resin and an organic peroxide, and a second component including (meth)acrylate polymerizable resin and a reducing agent, a photopolymerizaton initiator and a filler are included in the first component and/or the second component, and oxide powder of a Group II element in the periodic table having an average particle diameter of 1 μm or less is blended in the first component and/or the second component.