A flow rate adjusting valve in which a needle valve is arranged so as to face a fluid passage of a main body, the flow rate adjusting valve comprising: a handle configured to enable a rotational operation to be performed manually; an electric motor configured to be operated remotely; and a motive power transmission mechanism configured to selectively switch between a rotational operating force of the handle and a driving force of the electric motor, and to transmit the force to the needle valve, wherein the switching is performed by causing the handle to be moved in a direction of an axis of rotation, wherein: the handle is configured to be moved along the axis of rotation of the handle in first and second directions that are mutually opposite to each other; when the handle is pulled and moved in the first direction, a motive power transmission path between the handle and the needle valve is made continuous, and a motive power transmission path between the electric motor and the needle valve is interrupted midway; and when the handle is pushed and moved in the second direction, the motive power transmission path between the handle and the needle valve is interrupted midway, and the motive power transmission path between the electric motor and the needle valve is made continuous, characterized in that each of the handle and a handle supporting member of the main body comprises splines (16d, 56g), the splines of the handle and the splines of the handle supporting member being configured to be mutually engaged; and when the handle is moved in the second direction from an end of movement in the first direction, the splines of the handle engage with the splines of the handle supporting member, whereby rotation of the handle is prevented. a handle configured to enable a rotational operation to be performed manually; an electric motor configured to be operated remotely; and a motive power transmission mechanism configured to selectively switch between a rotational operating force of the handle and a driving force of the electric motor, and to transmit the force to the needle valve, wherein the switching is performed by causing the handle to be moved in a direction of an axis of rotation, wherein: the handle is configured to be moved along the axis of rotation of the handle in first and second directions that are mutually opposite to each other; when the handle is pulled and moved in the first direction, a motive power transmission path between the handle and the needle valve is made continuous, and a motive power transmission path between the electric motor and the needle valve is interrupted midway; and when the handle is pushed and moved in the second direction, the motive power transmission path between the handle and the needle valve is interrupted midway, and the motive power transmission path between the electric motor and the needle valve is made continuous, characterized in that each of the handle and a handle supporting member of the main body comprises splines (16d, 56g), the splines of the handle and the splines of the handle supporting member being configured to be mutually engaged; and when the handle is moved in the second direction from an end of movement in the first direction, the splines of the handle engage with the splines of the handle supporting member, whereby rotation of the handle is prevented. the handle is configured to be moved along the axis of rotation of the handle in first and second directions that are mutually opposite to each other; when the handle is pulled and moved in the first direction, a motive power transmission path between the handle and the needle valve is made continuous, and a motive power transmission path between the electric motor and the needle valve is interrupted midway; and when the handle is pushed and moved in the second direction, the motive power transmission path between the handle and the needle valve is interrupted midway, and the motive power transmission path between the electric motor and the needle valve is made continuous, characterized in that each of the handle and a handle supporting member of the main body comprises splines (16d, 56g), the splines of the handle and the splines of the handle supporting member being configured to be mutually engaged; and when the handle is moved in the second direction from an end of movement in the first direction, the splines of the handle engage with the splines of the handle supporting member, whereby rotation of the handle is prevented.