The present invention relates to a handheld automatic injection device comprising a housing (10, 30, 40； 40&prime；), a piston rod (80) located within the housing (10, 30, 40； 40&prime；), a drive member (70) permanently coupled to the piston rod (80), a dose setting member (50), which during dose setting rotates relative to the housing (10, 30, 40； 40&prime；) in a first direction and which during dose dispensing rotates relative to the housing (10, 30, 40； 40&prime；) in a second opposite direction, and a power reservoir (100) for driving the drive member (70). The drive member (70) is axially movable between a dose setting position, in which the drive member (70) is rotationally constrained to the housing, and a dose dispensing position, in which the drive member (70) is rotationally decoupled from the housing (10, 30, 40； 40&prime；). The power reservoir comprises a reverse wound flat spiral spring (100) having a first end attached to a first spool (101) and a second end attached to a second spool (102), with one of the spools (102) being coupled to the drive member (70).
