PURPOSE: To obtain an AC generator for vehicle which can realize high speed rotation by optimizing the ratio between the inner diameter and the thickness of stator, in case of 16 poles, according to a specific formula.

CONSTITUTION: A rotor 1 is excited by a DC exciting current to be fed to a field winding 2 and rotates with N and S poles being produced alternately in claw poles 111 arranged on the outer periphery of rotor thus inducing an AC three-phase output in a stator coil 12. When the inner diameter Di and the lamination thickness L of stator core are set such that 0.20≤L/Di≤0.24 for 16 field poles, output/mass ratio is improved to allow high speed rotation. When Do=l28, for example, Di=95mm and L=20mm and thereby L/Di=0.21 thus producing an output substantially equivalent to that in case of 12 poles and realizing high speed operation upto 22000rpm.