Chaos shift keying (CSK) is a modulation method in digital communication systems using chaotically modulated signals. This paper proposes novel CSK which utilizes two types of chaotic synchronization called in-phase and anti-phase chaotic synchronization. In this method, binary signals are mapped into two phases of chaotic synchronization, and a transmitter generates a two-phase-shift-keyed chaotic signal. So it will be called chaotic phase shift keying (CPSK) in this paper. This method is simpler than that based on two pairs of different chaotic systems. We also discuss an effect of noise in transmission lines.