It worked out by setting "mni152reg" option as True - exactly as described as here. Thanks!
By the way, I found out an example registration file, which looks like this:
$ cat mni152.register.dat
9.975314e-01 -7.324822e-03 1.760415e-02 9.570923e-01
-1.296475e-02 -9.262221e-03 9.970638e-01 -1.781596e+01
-1.459537e-02 -1.000945e+00 2.444772e-03 -1.854964e+01
0 0 0 1
With this file, it worked the same way as setting "mni152reg" option as True. But, don't really know what this is....