The present invention provides a catalyst composition and a method for producing an olefin polymer using the catalyst composition. The catalyst composition is prepared by bringing (A) a transition metal compound, (B) a solid boron compound capable of forming an ion pair with the component (A), (C) an organometallic compound and (D) a compound represented by the following general formula (XIV) and/or the following general formula (XV) into contact with each other in a hydrocarbon solvent, and enables a high catalyst concentration. Z5R14R15  (XIV) Z6R14R15R16  (XV) [In the formulae, Z5 represents an oxygen atom, etc.; Z6 represents a nitrogen atom, etc.; R14 to R16 each independently represent an organic group, and at least one of those organic groups is an organic group having at least 3 carbon atoms, and R14 to R16 may bond to each other to form a ring.]