  The mid-infrared range is an important spectrum range where materials exhibit
a characteristic response corresponding to their molecular structure. A
free-electron laser (FEL) is a promising candidate for a high-power light
source with wavelength tunability to investigate the nonlinear response of
materials. Although the self-amplification spontaneous emission (SASE) scheme
is not usually adopted in the mid-infrared wavelength range, it may have
advantages such as layout simplicity, the possibility of producing a single
pulse, and scalability to a short-wavelength facility. To demonstrate the
operation of a mid-infrared SASE FEL system in an energy recovery linac (ERL)
layout, we constructed an SASE FEL setup in cERL, a test facility of the
superconducting linac with the ERL configuration. Despite the adverse
circumstance of space charge effects due to the given boundary condition of the
facility, we successfully established the beam condition at the undulators, and
observed FEL emission at a wavelength of 20 $\mu$m. The results show that the
layout of cERL has the potential for serving as a mid-infrared light source.
