A safety switch input diagnosis device, comprising a switch unit and a transmission unit, wherein the safety switch input diagnosis device comprises: in said switch unit a first circuit having one end and another end and formed from a series connection of a switch connected to the other end of the first circuit and capable of interrupting connection between a first contact and a second contact and a line having a first resistor connected between the second contact of the switch and the one end of the first circuit; in said switch unit a second circuit having a second resistor between one end and another end and connected at the one end to the one end of the first circuit and at the other end to a first connector of said switch unit; a third circuit being a line connected at one end to the first connector and at another end to a fourth connector of said transmission unit connected to a power supply (Vcc); a fourth circuit being a line connected at one end to a third connector of said switch unit connected to the other end of the first circuit; in said transmission unit a fifth circuit having a third resistor between one end and another end and connected at the one end to a sixth connector of said transmission unit connected to the other end of the fourth circuit and at the other end to a ground; in said switch unit a sixth circuit having a fourth resistor between one end and another end and connected at the one end to the one end of the first circuit and at the other end to the other end of the first circuit; a seventh circuit being a line connected at one end to a second connector of said switch unit connected to the one end of the first circuit; in said transmission unit an eighth circuit having a fifth resistor between one end and another end and connected at the one end to a fifth connector of said transmission unit connected to the other end of the seventh circuit and at the other end to the ground; and in said transmission unit a controller connected to the one end of the eighth circuit and configured to diagnose a connection state of the switch and failure modes of the third, the fourth, and the seventh circuits on a basis of a voltage value at the one end of the eighth circuit and to output a result of diagnosis. in said switch unit a first circuit having one end and another end and formed from a series connection of a switch connected to the other end of the first circuit and capable of interrupting connection between a first contact and a second contact and a line having a first resistor connected between the second contact of the switch and the one end of the first circuit; in said switch unit a second circuit having a second resistor between one end and another end and connected at the one end to the one end of the first circuit and at the other end to a first connector of said switch unit; a third circuit being a line connected at one end to the first connector and at another end to a fourth connector of said transmission unit connected to a power supply (Vcc); a fourth circuit being a line connected at one end to a third connector of said switch unit connected to the other end of the first circuit; in said transmission unit a fifth circuit having a third resistor between one end and another end and connected at the one end to a sixth connector of said transmission unit connected to the other end of the fourth circuit and at the other end to a ground; in said switch unit a sixth circuit having a fourth resistor between one end and another end and connected at the one end to the one end of the first circuit and at the other end to the other end of the first circuit; a seventh circuit being a line connected at one end to a second connector of said switch unit connected to the one end of the first circuit; in said transmission unit an eighth circuit having a fifth resistor between one end and another end and connected at the one end to a fifth connector of said transmission unit connected to the other end of the seventh circuit and at the other end to the ground; and in said transmission unit a controller connected to the one end of the eighth circuit and configured to diagnose a connection state of the switch and failure modes of the third, the fourth, and the seventh circuits on a basis of a voltage value at the one end of the eighth circuit and to output a result of diagnosis.