To determine abnormal conditions of an element contained in a vehicle drive circuit or detect an abnormal element from a plurality of elements for the vehicle drive circuit, distinctly from other abnormal factors.

At each time interval Δt, a difference between a temperature change measured value ΔTm of a switching element and a temperature change estimated value ΔTe is determined as a measured value deviation D (n), and further, a measured value deviation-time change rate A as a time change rate thereof is determined. Temperature change is a value obtained by subtracting a temperature reference value from the temperature of the switching element. The temperature change measured value ΔTm is determined by subtracting the temperature reference value from a temperature detected value of the switching element, and the temperature change estimated value ΔTe is determined on the basis of a current passing in the switching element. The measured value deviation-time change rate A is added to a determination integration value SUM every time one processing set is performed. If the determination integration value SUM is larger than a threshold β, it is determined that heat resistance deterioration is caused in the switching element.