A write power adjustment method and optical read/write apparatus for accurate optimum write power setup. Different levels of write power Pwm are used to perform a test write and calculate a modulation Mm corresponding to each level of write power Pwm. Different write power compensated values Pcn are used to calculate an evaluated value Smn=Mm×(Pwm−Pcn). Pcn that maximizes the linearity of the relationship between compensated write power (Pwm−Pcn) and evaluated value Smn is determined as an optimum write power compensated value PcO. Compensated write power PthO that provides a modulation of zero, that is, an evaluated value SmO of zero, when the relationship between compensated write power (Pwm−PcO) for PcO and evaluated value SmO is subjected to linear approximation is determined. PthO and optimum write power compensated value PcO are used to calculate a write power threshold Pth=PthO+PcO. A predetermined calculation is performed on Pth to determine optimum write power Popt. In contrast to a conventional write power adjustment method (κ method), the present invention makes it possible to accurately adjust for the optimum write power even when the optical disc's write power Pwm-to-evaluated value M×Pw relationship is not linear or its modulation M varies.