A collision avoidance system comprising: a driver state detection unit configured to detect a state of a driver of a host vehicle; an environment detection unit configured to detect an environment in which the host vehicle is traveling; an external warning device configured to perform an external warning operation that warns another vehicle traveling in the vicinity of the host vehicle; an internal warning device configured to perform an internal warning operation that warns the driver of the host vehicle; and a control unit configured to control the external warning device and the internal warning device, characterized in that - the control unit configured to control the external warning device to perform the external warning operation and to control the internal warning device to perform the internal warning operation so as to perform the internal warning operation at a predetermined time period after the external warning operation has been performed when the state of the driver detected by the driver state detection unit satisfies a driver's condition that is of a drowsing state of the driver, a state of the driver looking aside or a state of a seizure caused by a medical condition of the driver; and to control the external warning device and the internal warning device not to perform the external warning operation and the internal warning operation when the state of the driver detected by the driver state detection unit does not satisfy any one of the said driver's condition; and - the control unit configured to set a longer time period as the predetermined time period when the environment detected by the environment detection unit satisfies a state of a road surface indicating that a breaking distance may be longer on the road surface on which the host vehicle is travelling, than that of a case the environment does not satisfy the said state of the road surface. a driver state detection unit configured to detect a state of a driver of a host vehicle; an environment detection unit configured to detect an environment in which the host vehicle is traveling; an external warning device configured to perform an external warning operation that warns another vehicle traveling in the vicinity of the host vehicle; an internal warning device configured to perform an internal warning operation that warns the driver of the host vehicle; and a control unit configured to control the external warning device and the internal warning device, characterized in that - the control unit configured to control the external warning device to perform the external warning operation and to control the internal warning device to perform the internal warning operation so as to perform the internal warning operation at a predetermined time period after the external warning operation has been performed when the state of the driver detected by the driver state detection unit satisfies a driver's condition that is of a drowsing state of the driver, a state of the driver looking aside or a state of a seizure caused by a medical condition of the driver; and to control the external warning device and the internal warning device not to perform the external warning operation and the internal warning operation when the state of the driver detected by the driver state detection unit does not satisfy any one of the said driver's condition; and - the control unit configured to set a longer time period as the predetermined time period when the environment detected by the environment detection unit satisfies a state of a road surface indicating that a breaking distance may be longer on the road surface on which the host vehicle is travelling, than that of a case the environment does not satisfy the said state of the road surface. - the control unit configured to control the external warning device to perform the external warning operation and to control the internal warning device to perform the internal warning operation so as to perform the internal warning operation at a predetermined time period after the external warning operation has been performed when the state of the driver detected by the driver state detection unit satisfies a driver's condition that is of a drowsing state of the driver, a state of the driver looking aside or a state of a seizure caused by a medical condition of the driver; and to control the external warning device and the internal warning device not to perform the external warning operation and the internal warning operation when the state of the driver detected by the driver state detection unit does not satisfy any one of the said driver's condition; and - the control unit configured to set a longer time period as the predetermined time period when the environment detected by the environment detection unit satisfies a state of a road surface indicating that a breaking distance may be longer on the road surface on which the host vehicle is travelling, than that of a case the environment does not satisfy the said state of the road surface.