A digital beam forming apparatus in an ultrasound system having an array transducer containing a plurality of transducer elements, comprising: a variable sampling clock generating unit with a sampling clock generator configured to variably generate sampling clock signals based on reception delay times in ultrasound echo signals arriving at each element of the transducer elements of the array transducer, the variable sampling clock generating unit further being configured to output control signals corresponding to cycles of the sampling clock signals; sigma-delta analog-to-digital (A/D) converters connected to the respective transducer elements of the array transducer and being configured to convert analog signals outputted from the elements of the array transducer into digital signals in response to the sampling clock signals and the control signals; delay units configured to delay output signals of the sigma-delta A/D converters based on the reception delay time; and an adder configured to add the delayed signals, characterized in that each of the sigma-delta A/D converters includes an integrator for sampling the analog signal outputted from the transducer element of the array transducer in response two the sampling clock signals, the integrator having a variable capacitor whose capacitance is varied according to the control signals and an OP-Amp connected to the variable capacitor, wherein the capacitance of the variable capacitor is varied to have a larger value if the circle of the sampling clock signal corresponding to the control signal becomes shorter and the capacitance of the variable capacitor is varied to have a smaller value if the cycle of the sampling clock signal corresponding to the control signal becomes longer. a variable sampling clock generating unit with a sampling clock generator configured to variably generate sampling clock signals based on reception delay times in ultrasound echo signals arriving at each element of the transducer elements of the array transducer, the variable sampling clock generating unit further being configured to output control signals corresponding to cycles of the sampling clock signals; sigma-delta analog-to-digital (A/D) converters connected to the respective transducer elements of the array transducer and being configured to convert analog signals outputted from the elements of the array transducer into digital signals in response to the sampling clock signals and the control signals; delay units configured to delay output signals of the sigma-delta A/D converters based on the reception delay time; and an adder configured to add the delayed signals, characterized in that each of the sigma-delta A/D converters includes an integrator for sampling the analog signal outputted from the transducer element of the array transducer in response two the sampling clock signals, the integrator having a variable capacitor whose capacitance is varied according to the control signals and an OP-Amp connected to the variable capacitor, wherein the capacitance of the variable capacitor is varied to have a larger value if the circle of the sampling clock signal corresponding to the control signal becomes shorter and the capacitance of the variable capacitor is varied to have a smaller value if the cycle of the sampling clock signal corresponding to the control signal becomes longer.