  Radio observations of low-mass star formation in molecular spectral lines
have rapidly progressed since the advent of Atacama Large
Millimeter/submillimeter Array (ALMA). A gas distribution and its kinematics
within a few 100s au scale around a Class 0-I protostar are spatially resolved,
and the region where a protostellar disk is being formed is now revealed in
detail. In such studies, it is essential to characterize the complex physical
structure around a protostar consisting of an infalling envelope, a
rotationally-supported disk, and an outflow. For this purpose, we have
developed a general-purpose computer code `{\tt FERIA}' (Flat Envelope model
with Rotation and Infall under Angular momentum conservation) generating the
image cube data based on the infalling-rotating envelope model and the
Keplerian disk model, both of which are often used in observational studies. In
this paper, we present the description and the usage manual of {\tt FERIA} and
summarize caveats in actual applications. This program outputs cube {\tt FITS}
files, which can be used for direct comparison with observations. It can also
be used to generate mock data for the machine/deep learnings. Examples of these
applications are described and discussed to demonstrate how the model analyses
work with actual observational data.
