This paper aims to clarify how contingent convertible bond (CoCo) as a debt financing instrument affects a firm's investment policy, agency cost of debt, and capital structure. We consider endogenous and exogenous conversion thresholds, respectively. Under the exogenous case, there is an explicit optimal fraction of equity allocated to CoCo holders upon conversion, such that the agency cost reaches zero. Numerical analysis demonstrates that under an endogenous conversion threshold, CoCo induces overinvestment, a higher leverage, a possible bigger agency cost, and a stronger incentive to increase risk. But if the conversion threshold is exogenously determined, almost the opposite holds true.