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 . <EOS>