  We report on infrared imaging observations of the z=2.429 radio galaxy B3
0731+438 with the Subaru telescope. The images were taken with the K'-band
filter and the 2.25 um narrow-band filter to examine the structure and
properties of the Ha+[N II] 6548,6583 emission-line components. The Ha+[N II]
emission-line image shows biconical lobes with an extent of 40 kpc, which are
aligned with the radio axis. The rest-frame equivalent widths of the emission
lines at these cones are as large as 1100 AA, and can be well explained by a
gas-cloud model photoionized by power-law continuum radiation. The isotropic
ionizing photon luminosity necessary to ionize the hydrogen gas in these cones
amounts to 1e57(photons/s), which is larger than that in the majority of
radio-loud QSOs. From these results, we propose that the Ha alignment effect in
this object is produced by biconical gas clouds, which are swept up by the
passage of radio jets, and are ionized by strong UV radiation from a hidden
AGN. The continuum image consists of two components, a stellar-like point
source and an extended diffuse galaxy. These are supposed to be a type-2 AGN
and its host galaxy. The SED is fitted by a combination of spectra of a
reddened dust-scattered AGN and an instantaneous starburst population of 500
Myr old. The stellar mass of the galaxy is estimated to be 3e11 M_solar, which
is as large as that of typical 3C radio galaxies at z=1.
