Neutral beam injection (NBI) plasmas have been initiated with the assistance of 5 kW, 2.45 GHz microwaves in a medium-sized heliotron, Heliotron J plasmas with a line-averaged electron density of over 1 × 1019 m−3 are generated with 1 MW NBI using a magnetic field between 0.63 and 1.25 T within 20 ms after turning on NBI. This technique does not require the electron cyclotron resonance layer for the plasma startup, which enables us to enlarge the operational space for the magnetic field strength. Electron cyclotron emission measured with a radiometer reveals that the condition of initial plasmas generated by 2.45 GHz microwaves is critical for reliable startup.