The accelerator opening degree detection apparatus of the present invention includes: an accelerator pedal position sensor for detecting a position of an accelerator pedal; and a calculating section for calculating an accelerator opening degree based on an output of the accelerator pedal position sensor. The calculating section performs at least the following steps of: obtaining a signal PDLAD by smoothing the output of the accelerator pedal position sensor with a first smoothing coefficient; obtaining a signal PDLSM by smoothing the output of the accelerator pedal position sensor with a second smoothing coefficient which is larger than the first smoothing coefficient; storing a reference value GPDL representing a reference position of the accelerator pedal position sensor; calculating an accelerator opening degree PDLA based on a difference between the signal PDLAD and the reference value GPDL; and updating the reference value GPDL based on the signal PDLSM if the accelerator opening degree PDLA satisfies a predetermined update condition.
