We develop a new type of contingent capital, called contingent convertible security (CCS), which is like a contingent convertible bond (CCB) but differently can be interconverted \emph{repeatedly} and automatically between debt and equity depending on two specified levels of the cash flow generated by the firm that issues the CCS. We derive closed-form expressions of the equilibrium prices of corporate securities and optimal capital structure when the cash flow of the firm is modeled as geometric Brownian motion. Especially, we provide very simple formulas on optimal capital structure including a CCB. We show that the CCS can not only decrease default risk like a CCB, but also can significantly increase the firm's value. In particular, the CCS can be used to solve financing problems of small- and medium-sized enterprises as well as banks.