A reliable diagnostic device for free-electron lasers (FELs) is essential for both users and the evaluation of FEL sources and beamline optics. Here, we propose a compact bolometric radiometer (CBR) that can operate at room temperature. The CBR is mainly designed for FELs in the wavelength range from the extreme-ultraviolet (EUV) to x-rays; moreover, we confirmed that the proposed device can also be applied to synchrotron radiation. As an application of the CBR, we evaluated an FEL beamline transmission by means of bolometric technique in the EUV range for the first time, to the best of our knowledge. This indicates that the CBR is an effective diagnostic device for FELs.