Typical liquid-cooled compressors use the effective means of reducing no-load power by repeatedly starting and stopping an electric motor according to the amount of required air, but sufficient consideration has not been given to the fact that frequent starting and stopping of large-output electric motors leads to a decline in motor reliability. In order to solve this problem, a liquid-cooled compressor for circulating a liquid inside a compressor body using a pressure difference, and equipped with a cooling channel for circulating said liquid for cooling, configured so as to have an intake valve for adjusting the air intake of the compressor body, to change the amount of air taken in through the intake valve, and as a result, to perform a low-pressure operation during no-load operation at two levels of reduced operating pressure consisting of a value no less than a minimum circulation oil supply pressure and a low value. As a result, possible to provide a compressor which balances ensuring the reliability of the compressor and the electric motor during no-load operation of a large-output electric motor, and improving energy efficiency during no-load operation by reducing surplus power.