Several types of diagnostic system for soft X-ray radiation (SXR) have been developed and installed on the Large Helical Device (LHD). Magnetohydrodynamic (MHD) activities are studied by the fluctuation component of SXR, and the electron temperature is measured from the spectrum of the SXR. PIN photodiode arrays are installed at three poloidal cross sections to study the equilibrium and fluctuations. The spatial resolution of an array is a few centimeters, and its frequency response is up to [approximately]300 kHz. Absolute extreme ultraviolet diode arrays are also used for lower-energy radiation, and edge MHD instabilities are thereby studied. There are tangentially viewing two-dimensional soft X-ray camera systems by which more detailed spatial structure can be studied. Radial profile of X-ray spectrum is measured by X-ray pulse height analyzer systems.