We are developing a frequency-division multiplexing (FDM) readout system aimed to
realize the 400-pixel TES microcalorimeter array for the DIOS mission as well as
large-format arrays with more than a thousand of TES for future space missions such as
the ATHENA mission. The system consists of a low-power superconducting quantum
interference device (SQUID), a digital flux-locked loop (FLL) electronics, and an analog
front-end to bridge the SQUID and the FLL electronics. Using the developed readout
system, we performed a TES readout experiment, and succeeded to multiplex TES pixels
with a simple single-staged cryogenic setup. In this talk, I will review astrophysical
motivations, TES (and some non-TES) microcalorimeters, SQUIDs, multiplexing readout
techniques, the result from the readout experiment, and future prospects.