Abstract: A simple attachment structure of the present invention ensures favorable connection of a battery pack with a battery holder in a power-driven tool over a long time period without causing any defective contact. A pair of side slots (47a,47b) as well as a pair of insertion slots (48a,48b) are formed in a connection surface (49) of an upper battery case (20) of a battery pack (10). Electrode terminal assemblies (41a,41b) are set in the side slots (47a,47b) and the insertion slots (48a,48b) to be exposed on the two side faces thereof. A positive spring terminal (64a) and a negative spring terminal (64b) are integrally joined with and supported by a terminal base 61 of a battery holder (60). Each spring terminal (64a,64b) has an insertion element (66a,66b) formed to have a width a little greater than the width of the insertion slot (48a,48b).