A high pressure extraction apparatus and extracting method therefor are adapted to maintain a medium at the supercritical state to extract substance-to-be-extracted. The apparatus includes an extraction tank, a piston and a control unit. The control unit controls the piston to reciprocate in the extraction tank as well as controls the pressure of the extract space of the tank, feeding of the substance, discharge of extract, and discharge of leftover. Accordingly, the apparatus performs a continuous supercritical extraction.