A hydraulic circuit of an option device for an excavator is disclosed, which can constantly supply hydraulic fluid to an option device (24), such as a breaker and so on, selectively mounted on the excavator, irrespective of the size of a load occurring when the option device operates, and control respective flow rates required for various kinds of option devices. The hydraulic circuit includes a variable hydraulic pump (26), an option device (24), a first spool (15) shifted to control hydraulic fluid fed to the option device (24), a poppet (14) and a piston (13), an option spool (25) shifted to control hydraulic fluid fed to the option device (24) via the first spool (15), a second spool (3) shifted to control hydraulic fluid fed to a back pressure chamber (17) of the poppet (14), and a control means (14-2) installed in the poppet (14) and controlling hydraulic fluid passing through an orifice (14a) of the poppet (14) when the piston (13) and the poppet (14) are pressed by the hydraulic fluid fed from the hydraulic pump (26), through the shifting of the second spool (3).