A steering angle detecting apparatus for a vehicle comprising: a relative steering angle detecting unit including a steering angle sensor, the steering angle sensor continuously detecting a relative steering angle of a steering mechanism for steering steered wheels of the vehicle during a system startup while an ignition switch is in an on state and during a system shutdown while the ignition switch is in an off state; an absolute steering angle storage unit including a non-volatile memory in which a steering angle detection value of the steering mechanism outputted immediately before the system shutdown is stored as an absolute steering angle stored value; a provisional absolute steering angle value calculating unit configured to set, immediately after the system startup begins, a value obtained by correcting the absolute steering angle stored value stored in the non-volatile memory with a variation in the relative steering angle of the steering mechanism detected by the relative steering angle detecting unit from a last system shutdown until a present system startup, as an initial value of an absolute steering angle of the steering mechanism, and to calculate a provisional absolute steering angle value on the basis of the relative steering angle detected by the relative steering angle detecting unit, using the initial value of the absolute steering angle as a reference so as to output the provisional absolute steering angle value, during the system startup; is characterised by a relative steering angle storage unit including the non-volatile memory in which the relative steering angle detected by the relative steering angle detecting unit is stored as a relative steering angle stored value immediately before the system shutdown, wherein the provisional absolute steering angle value calculating unit adds a difference between the relative steering angle stored value stored in the non-volatile memory and the relative steering angle detected by the relative steering angle detecting unit to the absolute steering angle stored value stored in the non-volatile memory so as to calculate the provisional absolute steering angle value immediately after the system startup begins. a relative steering angle detecting unit including a steering angle sensor, the steering angle sensor continuously detecting a relative steering angle of a steering mechanism for steering steered wheels of the vehicle during a system startup while an ignition switch is in an on state and during a system shutdown while the ignition switch is in an off state; an absolute steering angle storage unit including a non-volatile memory in which a steering angle detection value of the steering mechanism outputted immediately before the system shutdown is stored as an absolute steering angle stored value; a provisional absolute steering angle value calculating unit configured to set, immediately after the system startup begins, a value obtained by correcting the absolute steering angle stored value stored in the non-volatile memory with a variation in the relative steering angle of the steering mechanism detected by the relative steering angle detecting unit from a last system shutdown until a present system startup, as an initial value of an absolute steering angle of the steering mechanism, and to calculate a provisional absolute steering angle value on the basis of the relative steering angle detected by the relative steering angle detecting unit, using the initial value of the absolute steering angle as a reference so as to output the provisional absolute steering angle value, during the system startup; is characterised by a relative steering angle storage unit including the non-volatile memory in which the relative steering angle detected by the relative steering angle detecting unit is stored as a relative steering angle stored value immediately before the system shutdown, wherein the provisional absolute steering angle value calculating unit adds a difference between the relative steering angle stored value stored in the non-volatile memory and the relative steering angle detected by the relative steering angle detecting unit to the absolute steering angle stored value stored in the non-volatile memory so as to calculate the provisional absolute steering angle value immediately after the system startup begins.