A diagnosis device for use with a fan coupling device which connects an internal combustion engine to a cooling fan that blows cooling air onto a radiator for cooling a coolant of the internal combustion engine, the fan coupling device comprising: an input shaft that is driven to rotate by the internal combustion engine; a fluid coupling that transmits torque from the input shaft to the cooling fan via a fluid; and a solenoid valve that adjusts a fluid amount in the fluid coupling, the solenoid valve comprising a valve main body that is displaced in response to energization between a lifted position in which the fluid amount increases and a closed position in which the fluid amount decreases, wherein the diagnosis device is used to determine whether or not the valve main body is stuck in the lifted position, the diagnosis device comprising: a sensor that detects a rotation speed ( Nfan) of the cooling fan; and a controller programmed to: output a signal in order to return the valve main body to the closed position; and characterised in that the controller is further programmed to start to determine whether or not the valve main body is stuck in the lifted position on the basis of the rotation speed ( Nfan) of the cooling fan following the elapse of a predetermined time from output of the signal (S7-S10). an input shaft that is driven to rotate by the internal combustion engine; a fluid coupling that transmits torque from the input shaft to the cooling fan via a fluid; and a solenoid valve that adjusts a fluid amount in the fluid coupling, the solenoid valve comprising a valve main body that is displaced in response to energization between a lifted position in which the fluid amount increases and a closed position in which the fluid amount decreases, wherein the diagnosis device is used to determine whether or not the valve main body is stuck in the lifted position, the diagnosis device comprising: a sensor that detects a rotation speed ( Nfan) of the cooling fan; and a controller programmed to: output a signal in order to return the valve main body to the closed position; and characterised in that the controller is further programmed to start to determine whether or not the valve main body is stuck in the lifted position on the basis of the rotation speed ( Nfan) of the cooling fan following the elapse of a predetermined time from output of the signal (S7-S10). a sensor that detects a rotation speed ( Nfan) of the cooling fan; and a controller programmed to: output a signal in order to return the valve main body to the closed position; and characterised in that the controller is further programmed to start to determine whether or not the valve main body is stuck in the lifted position on the basis of the rotation speed ( Nfan) of the cooling fan following the elapse of a predetermined time from output of the signal (S7-S10). output a signal in order to return the valve main body to the closed position; and characterised in that the controller is further programmed to start to determine whether or not the valve main body is stuck in the lifted position on the basis of the rotation speed ( Nfan) of the cooling fan following the elapse of a predetermined time from output of the signal (S7-S10).