The experimental results of ATC (Adiabatic Toroidal Compressor) are re-analysed with magnetic-compression theory for clarifying the inductance of compressed plasma. Its time-varying nature during compression is revealed, as there has been uncertainty since 1977 (Daughney et al 1977 Nucl. Fusion17 2). During compression in the major radius, the plasma inductance decreases quasi-linearly with the major radius, and its magnetic energy increases quasi-linearly with the major radius.