A polymerization initiator composition which does not burn or ignite even when it contacts paper or the like in air and can provide high polymerization activity to a polymerizable composition to cure it in a short period of time, and an adhesive composition comprising the same. The above polymerization initiator composition comprises 100 parts by weight of (A) an organic boron compound such as a trialkyl boron, alkoxyalkyl boron, dialkyl borane, monocyclo or bicyclo compound having a boron atom as a hetero atom or partially oxidated trialkyl boron, 5 to 40 parts by weight of (B) an aprotic solvent having a boiling point of 50 to 120° C, and 0.2 to 5 parts by weight of (C) an alcohol having a boiling point of 60 to 180°C.
