To provide an installation structure of a battery pack of a hybrid vehicle and an electric automobile for arranging a battery between side members.

The installation structure of the battery pack includes a first bracket 3 of applying bends of a plurality of upward projection shapes and a second bracket 8 of applying bends of a plurality of upward recessed shapes to positions corresponding to the bends of the first bracket 3, a plurality of closed cross sections are formed by joining a part applied with the bends of the projection shapes of the first bracket 3 and a part applied with the bend of the second bracket 8 in the vertical direction, and an insertion hole of a battery pack installing bolt 5 is formed between the plurality of closed cross sections.