We have investigated intersublevel transitions in single self-assembled InAs quantum dots (QDs) by using a single electron transistor geometry that consists of a QD and nanogap metal electrodes as a terahertz detector. Photocurrent distribution with respect to the Coulomb diamonds indicates that there are two mechanisms for the photocurrent generation. When the p shell was fully occupied, we observed rather simple photocurrent spectra induced by the p->d transitions. However, when the p shell was half-filled, the photocurrent spectra exhibited a complicated behavior, which we attribute to the fluctuation in electron configuration when the empty p state is filled back from the electrodes.