  We report the results of 27Al-NMR measurements on a single crystal of
PrTi2Al20. This compound shows a phase transition near 2 K associated with the
quadrupole degree of freedom of Pr ions, for which the ground state of the
crystalline electric field is a nonmagnetic doublet. When a magnetic field is
applied along the <111> direction, the NMR lines split upon entering the
low-temperature phase, indicating the breaking of the three fold rotation
symmetry due to the field-induced magnetic dipole perpendicular to the field.
This provides microscopic evidence for a ferro order of the O20 quadrupole.
Although a first-order transition is expected theoretically, the line splitting
evolves continuously within the experimental resolution.
