A preventive apparatus that includes a switch (21, 22) provided on an electric current path, and that is configured to prevent an overcurrent from flowing through the electric current path by turning the switch (21, 22) off, the preventive apparatus comprising: a resistor (R1, R2) whose one end is connected to one end of the switch (21, 22); a constant current circuit (25, 28) for causing a constant current (I1, I2) to flow from the one end side of the resistor (R1, R2) to the other end side of the resistor (R1, R2); changing means for changing a value of the current (I1, I2) that is caused to flow by the constant current circuit (25, 28); and a comparator (26, 29) for comparing an electric potential at the other end of the switch (21, 22) with an electric potential at the other end of the resistor (R1, R2), wherein the preventive apparatus is configured to turn off the switch if a result of the comparison performed by the comparator (26, 29) shows that the electric potential at the other end of the switch (21, 22) is lower than the electric potential at the other end of the resistor (R1, R2), characterized in that the constant current circuit (25, 28) includes a current mirror circuit provided with at least one transistor, the at least one transistor has a first terminal, a second terminal, and a third terminal, in the at least one transistor, a current having a value that corresponds to a voltage between the first terminal and the second terminal of the transistor flows between the second terminal and the third terminal of the transistor, the second terminal of the at least one transistor is grounded, the other end of the resistor (R1, R2) is connected to the third terminal of the at least one transistor, and the changing means is configured to change the value of the current (I1, I2) that is caused to flow by the constant current circuit (25, 28) through the resistor (R1, R2), which is the current flowing between the second and third terminal of the transistor that is dependent on the voltage between the first terminal and the second terminal of the transistor, by changing the value of a current (Iref) flowing in the current mirror circuit on the first terminal side of the at least one transistor. a resistor (R1, R2) whose one end is connected to one end of the switch (21, 22); a constant current circuit (25, 28) for causing a constant current (I1, I2) to flow from the one end side of the resistor (R1, R2) to the other end side of the resistor (R1, R2); changing means for changing a value of the current (I1, I2) that is caused to flow by the constant current circuit (25, 28); and a comparator (26, 29) for comparing an electric potential at the other end of the switch (21, 22) with an electric potential at the other end of the resistor (R1, R2), wherein the preventive apparatus is configured to turn off the switch if a result of the comparison performed by the comparator (26, 29) shows that the electric potential at the other end of the switch (21, 22) is lower than the electric potential at the other end of the resistor (R1, R2), characterized in that the constant current circuit (25, 28) includes a current mirror circuit provided with at least one transistor, the at least one transistor has a first terminal, a second terminal, and a third terminal, in the at least one transistor, a current having a value that corresponds to a voltage between the first terminal and the second terminal of the transistor flows between the second terminal and the third terminal of the transistor, the second terminal of the at least one transistor is grounded, the other end of the resistor (R1, R2) is connected to the third terminal of the at least one transistor, and the changing means is configured to change the value of the current (I1, I2) that is caused to flow by the constant current circuit (25, 28) through the resistor (R1, R2), which is the current flowing between the second and third terminal of the transistor that is dependent on the voltage between the first terminal and the second terminal of the transistor, by changing the value of a current (Iref) flowing in the current mirror circuit on the first terminal side of the at least one transistor.