A safety-extension base connectable to a main base in a programmable logic controller system, the safety-extension base comprising: a communication unit configured to perform data communication with the main base; a system state monitoring unit configured to monitor a state of the main base, through its communication with the main base, using data communicated between the communication unit and the main base; a module control unit configured to control a general operation of the safety-extension base and output a different case of control signals according to whether or not the state of the main base is a normal state; a memory unit configured to store the data transmitted from the main base and a program and information, necessary for controlling the safety state of the safety-extension base; a state conversion unit configured to convert the state of the safety-extension base into the normal state or safety state according to the control signal output from the module control unit; and a safety-processing unit configured to perform a control of the safety-extension base using the information transmitted from the main base in a case where the state of the safety-extension base is converted into the normal state by the state conversion unit, and perform a control of the safety-extension base according to a user-defined program previously stored in the memory unit in a case where the state of the safety-extension base is converted into the safety state by the state conversion unit, wherein the safety-extension base is characterized in that when the safety-extension base is converted into the safety state, the safety-extension base maintains and controls general extension bases separately from the main base, wherein the general extension bases are connected to the main base of the programmable logic controller and are separate from and independent of the safety-extension base. a communication unit configured to perform data communication with the main base; a system state monitoring unit configured to monitor a state of the main base, through its communication with the main base, using data communicated between the communication unit and the main base; a module control unit configured to control a general operation of the safety-extension base and output a different case of control signals according to whether or not the state of the main base is a normal state; a memory unit configured to store the data transmitted from the main base and a program and information, necessary for controlling the safety state of the safety-extension base; a state conversion unit configured to convert the state of the safety-extension base into the normal state or safety state according to the control signal output from the module control unit; and a safety-processing unit configured to perform a control of the safety-extension base using the information transmitted from the main base in a case where the state of the safety-extension base is converted into the normal state by the state conversion unit, and perform a control of the safety-extension base according to a user-defined program previously stored in the memory unit in a case where the state of the safety-extension base is converted into the safety state by the state conversion unit, wherein the safety-extension base is characterized in that when the safety-extension base is converted into the safety state, the safety-extension base maintains and controls general extension bases separately from the main base, wherein the general extension bases are connected to the main base of the programmable logic controller and are separate from and independent of the safety-extension base.