An ATM exchange system capable of accommodation not only terminals of an ATM system but also terminals of an STM system at low cost. The ATM system includes an ATM channel switch for providing a direct exchange connection between the ATM terminal and an ATM trunk line, an STM channel switch for providing a direct exchange connection between the STM terminal and an STM trunk line, and an STM/ATM conversion module connected between the ATM channel switch and the STM channel switch and adapted to convert a call of one system to that of another system when a call is made between ATM terminal/line and STM terminal/line.