A primer used for pre-treating the surfaces of a prosthetic, which exhibits a large adhering force at the time of adhering a prosthetic of metal oxide ceramics to an object to be joined by using an adhesive such as a dental cement, the large adhering force being maintained for extended periods of time even in a severe environment such as in an oral cavity. A primer composition for metal oxide ceramics includes (A) a polymerizable monomer component containing 5% by mass or more, preferably, 5% by mass to 80% by mass of an acidic group-containing polymerizable monomer as represented by a 11-methacryloyloxy-1,1-undecanedicarboxylic acid and a rest of a polymerizable monomer without acidic group, such as triethylene glycol dimethacrylate； (B) polyvalent metal ions resulting from a titanium tetrabutoxide or the like； and (C) an organic solvent such as acetone or ethyl alcohol, wherein water is not substantially contained.
