A nebulizer capable of supplying oxygen gas of high concentration by increasing the flow rate of the oxygen gas and capable of forming minute aerosol irrespective of a change in the flow rate of the oxygen gas. A nebulizer (A) has a nozzle member (11) for discharging oxygen gas from an orifice (11b), an aerosol forming member (12) mounted at a position corresponding to the orifice, sucking water by suction created by flow of the oxygen gas discharged from the orifice, and forming the sucked water into minute aerosol, and a window (9) and a slit (10) which suck air as the oxygen gas is discharged from the orifice of the nozzle member. The nozzle member (11) has, in addition to the orifice (11b), an oxygen gas outflow section formed in a direction different from that of the orifice and causing supplied oxygen gas to flow thereout according to the pressure of the oxygen gas.