  We have observed the 3.8 s pulsar CXOU J171405.7-381031 with XMM-Newton, and
discovered the significant dP/dt of 6.40+/-0.14X10^-11 s/s from this source for
the first time, with the aid of archival Chandra data. The characteristic age
(950 yr), the magnetic field strength (5X10^14 G), and the spin-down luminosity
(4.5X10^34 erg/s) derived from P and dP/dt lead us to conclude that CXOU
J171405.7-381031 should be identified as a new magnetar. The obtained
characteristic age indicates that CXOU J171405.7-381031 is youngest among all
known anomalous X-ray pulsars, which is consistent with the age estimation from
the thermal X-rays of the associated supernova remnant. The ratio between 2-10
keV luminosity and spin-down luminosity is almost unity, which implies that
CXOU J171405.7-381031 is the key source to connect magnetars and traditional
radio pulsars.
