We have constructed an achromatic half wave plate ( AHWP ) suitable for the millimeter wavelength band .
The AHWP was made from a stack of three sapphire a-cut birefringent plates with the optical axes of the middle plate rotated by 50.5 degrees with respect to the aligned axes of the other plates .
The measured modulation efficiency of the AHWP at 110 GHz was 96 \pm 1.5 % .
In contrast , the modulation efficiency of a single sapphire plate of the same thickness was 43 \pm 4 % .
Both results are in close agreement with theoretical predictions .
The modulation efficiency of the AHWP was constant as a function of incidence angles between 0 and 15 degrees .
We discuss design parameters of an AHWP in the context of astrophysical broad band polarimetry at the millimeter wavelength band .