By Schwarzenberger's property, a complex vector bundle of dimension t over the complex projective space CP^n is extendible to CP^<n+k> for any k ≥ 0 if and only if it is stably equivalent to a Whitney sum of t complex line bundles. In this paper, we show some conditions for a negative multiple of a complex line bundle over CP^n to be extendible to CP^<n+1> or CP^<n+2>, and its application to unextendibility of a normal bundle of CP^n.