  We have observed the quadruply lensed quasar 1RXS J1131-1231 with the
integral field spectrograph mode of the Kyoto Tridimensional Spectrograph II
mounted on the Subaru telescope. Its field of view has covered simultaneously
the three brighter lensed images A, B, and C, which are known to exhibit
anomalous flux ratios in their continuum emission. We have found that the
[OIII] line flux ratios among these lensed images are consistent with those
predicted by smooth-lens models. The absence of both microlensing and
millilensing effects on this [OIII] narrow line region sets important limits on
the mass of any substructures along the line of sight, which is expressed as
M_E < 10^5 M_solar for the mass inside an Einstein radius. In contrast, the
H_beta line emission, which originates from the broad line region, shows an
anomaly in the flux ratio between images B and C, i.e., a factor two smaller
C/B ratio than predicted by smooth-lens models. The ratio of A/B in the H_beta
line is well reproduced. We show that the anomalous C/B ratio for the H_beta
line is caused most likely by micro/milli-lensing of image C. This is because
other effects, such as the differential dust extinction and/or arrival time
difference between images B and C, or the simultaneous lensing of another pair
of images A and B, are all unlikely. In addition, we have found that the broad
H_beta line of image A shows a slight asymmetry in its profile compared with
those in the other images, which suggests the presence of a small microlensing
effect on this line emitting region of image A.
