  We describe the observing simulation software FISVI (FIS Virtual Instrument),
which was developed for the Far-Infrared Surveyor (FIS) that will be on the
Japanese infrared astronomy mission ASTRO-F. The FISVI has two purposes: one is
to check the specifications and performances of the ASTRO-F/FIS as a whole; the
other is to prepare input data sets for the data analysis softwares prior to
launch. In the FISVI, special care was taken by introducing the "Compiled PSF
(Point Spread Function)" to optimise inevitable, but time-consuming,
convolution processes. With the Compiled PSF, we reduce the computation time by
an order of magnitude. The photon and readout noises are included in the
simulations. We estimate the detection limits for point sources from the
simulation of virtual patches of the sky mostly consisting of distant galaxies.
We studied the importance of source confusion for simple power-law models for
N(>S), the number of sources brighter than S. We found that source confusion
plays a dominant role in the detection limits only for models with rapid
luminosity evolution for the galaxy counts, the evolution of which is suggested
by recent observations.
