Abstract: A determining device for determining a failure in a motor compressor system, comprising a current detecting sensor for detecting a current input to a motor for driving a compressor, a mass flow sensor for detecting mass flow of compressible fluid sucked into the compressor, a compression ratio detecting sensor for detecting a compression ratio of the compressor, a reference value generating unit for generating a limit reference value for determining whether or not a failure in the motor compressor system has occurred by comparing the current detected by the current detecting sensor with the limit reference value, and a failure determining unit for determining that a failure in the motor compressor system has occurred when the current is beyond the limit reference value. The limit reference value is increased in accordance with an increase of the mass flow or an increase of the compression ratio.