We developed a magnetic suspension balance/diffusion-tube humidity generator (MSB/DTG) with the aim of establishing a primary humidity standard in the amount-of-substance fraction of moisture down to 10 nmol/mol. A comparison between humidity generated with the MSB/DTG and humidity measured with a moisture analyzer based on cavity ring-down spectroscopy was performed. It was demonstrated that the MSB/DTG is capable of steadily producing a trace-moisture gas down to 10 nmol/mol, and of quickly changing the amount-of-substance fraction of the gas generated.