The invention describes a method of inspecting an air-gap (1) of an electrical machine (2), which method comprises the steps of directing at least one laser beam (L) into the air-gap (1); detecting the laser beam (L) after reflection; determining the distance (d OK , d F ) travelled by the laser beam (L); and analysing the travelled distance (d OK , d F ) to detect an irregularity (F pole , F oval , F object ) in the air-gap (1). The invention further describes an air-gap inspection apparatus (3) for detecting an irregularity in the air-gap (1) of an electrical machine (2), which apparatus (3) comprises a laser module (30) realised to direct at least one laser beam (L) into the air-gap (1) and to detect the laser beam (L) after reflection; a computation unit (31) realised to determine the distance (d OK , d F ) travelled by the laser beam (L); and an analysis unit (32) realised to analyse the travelled distance (d OK , d F ) to detect an irregularity in the air-gap (1).