Disclosed are a method and a device for aligning shaft doors of an elevator, said device comprising a mounting gage (7) which can be mounted on guide rails (4) of an elevator car (1.1) and consists of a first part (7.1) to be mounted on one guide rail (4), a second part (7.2) to be mounted on the other guide rail (4), an electro-optical alignment instrument (8) for vertically aligning shaft door parts (11), and a set (12) of strings comprising at least two strings (12.1, 12.2) for horizontally aligning the shaft door parts (11).