An oil tank comprising: an intake port which is provided in the vicinity of a bottom of a tank body adapted to hold oil and which is connectable to a hydraulic pump adapted to supply hydraulic pressure oil to a hydraulic circuit; and a return oil inlet which is connectable to a return pipe of the hydraulic circuit and through which return oil from the hydraulic circuit can be discharged to the tank body, wherein a container adapted to hold the return oil is provided in the tank body, the container including: a bottom formed by the bottom of the tank body; an open end positioned above an upper operational oil level of stock oil held in the tank body; and the return oil inlet which opens in the vicinity of the bottom of the container, and a flow adjustment wing (424,425) is arranged along the open end; wherein the flow adjustment wing (424,425) is provided at a position below a lower operational oil level of stock oil held in the tank body so that a flow of stock oil held in the container and overflowing through the open end is converted, at a position below a lower operational oil level of the stock oil held in the tank body, to a flow along a surface of the stock oil held in the tank body through a channel formed between the flow adjustment wing and the surface of the stock oil in the tank body. an intake port which is provided in the vicinity of a bottom of a tank body adapted to hold oil and which is connectable to a hydraulic pump adapted to supply hydraulic pressure oil to a hydraulic circuit; and a return oil inlet which is connectable to a return pipe of the hydraulic circuit and through which return oil from the hydraulic circuit can be discharged to the tank body, wherein a container adapted to hold the return oil is provided in the tank body, the container including: a bottom formed by the bottom of the tank body; an open end positioned above an upper operational oil level of stock oil held in the tank body; and the return oil inlet which opens in the vicinity of the bottom of the container, and a flow adjustment wing (424,425) is arranged along the open end; wherein the flow adjustment wing (424,425) is provided at a position below a lower operational oil level of stock oil held in the tank body so that a flow of stock oil held in the container and overflowing through the open end is converted, at a position below a lower operational oil level of the stock oil held in the tank body, to a flow along a surface of the stock oil held in the tank body through a channel formed between the flow adjustment wing and the surface of the stock oil in the tank body. a bottom formed by the bottom of the tank body; an open end positioned above an upper operational oil level of stock oil held in the tank body; and the return oil inlet which opens in the vicinity of the bottom of the container, and a flow adjustment wing (424,425) is arranged along the open end; wherein the flow adjustment wing (424,425) is provided at a position below a lower operational oil level of stock oil held in the tank body so that a flow of stock oil held in the container and overflowing through the open end is converted, at a position below a lower operational oil level of the stock oil held in the tank body, to a flow along a surface of the stock oil held in the tank body through a channel formed between the flow adjustment wing and the surface of the stock oil in the tank body.