The charged particle beam writing apparatus includes a position deflection control circuit. First digital data that is to be used for circuit diagnosis is transmitted from the position deflection control circuit to the DAC amplifier unit at the same rate as a rate of writing on a product reticle and stored in a first maintenance memory. Second digital data is output from a digital section included in the DAC amplifier unit in response to the first digital data and stored in a second maintenance memory. A maintenance clock generator generates a clock signal and reads the first digital data stored in the first maintenance memory and the second digital data stored in the second maintenance memory. The first digital data thus read is compared with the second digital data thus read for each bit to diagnose the digital section.