The purpose of the present invention is to provide a fuel injection device and diesel engine whereby it is possible to reduce the amount of fuel oil supplied to a combustion chamber when a valve is closed. A fuel injection device (10) is provided with a housing (14), a needle valve (26), a spool valve (44) provided to a distal end of the needle valve (26), and a vertical groove (23) formed in a different position from fuel injection holes (16a-16d) and communicated with a fuel supply path (24), a fuel oil introduction space (27) through which fuel oil passes being formed in the spool valve (44), the fuel oil introduction space (27) being communicated with the vertical groove (23) and the fuel oil introduction space (27) moving to a position of communication with the fuel injection holes (16a-16d) when the fuel supply passage (24) is open, and an external peripheral face of the spool valve (44) confronting the fuel injection holes (16a-16d) and the fuel oil introduction space (27) moving to a position of non-communication with the fuel injection holes (16a-16d) when the fuel supply path (24) is closed.