The invention relates to a signal receiving and sending circuit of a protective circuit. The protective circuit comprises a sending circuit equipped in the high-voltage side of the protective circuit and applied for sending two-valued signals output from the high-voltage side of the protective circuit in a way to set the electric level of the two-valued signals within a range from a negative electrode voltage of a secondary battery connected to the high-voltage side of the protective circuit to a voltage range of the first voltage of the negative electrode voltage; an adjusting part equipped inside a low-voltage side of the protective circuit for adjusting and lowering the electrical level of the two-valued signals sent from the sending circuit through signal wirings within a range from a productive circuit low-voltage side positive voltage equal to the negative electrode voltage of the secondary battery connected to the high-voltage side of the protective circuit to a voltage of a second low voltage of the negative electrode voltage; and a detecting part equipped inside the low-voltage side of the protective circuit for detecting the breakage of the signal wiring when the adjusted two-valued signal is lower than the voltage range.