The diagnostic based on neutral beam injector DINA-5 is developed for the plasma density measurements at midplane of Gas Dynamic Trap (GDT1) experiment. The deuterium neutral beam with energy of 25 keV and equivalent current of up to 2 A is injected perpendicularly to plasma column at the midplane of the device. The beam is attenuated by 2–3 times passing through the plasma. The generated ions are deflected by the magnetic field and registered by a detector array located between the plasma and first wall. The deuterium ions produced in various points along the initial beam trajectory are detected in different channels. The signal in each detector depends on the local plasma density in corresponding point enabling to reconstruct the plasma density profile along the beam. In the experiments with the powerful neutral beam injection the plasma diamagnetism achieves considerable value (β ~ 20%) therefore it has to be taken into account for the accurate calculation of the D+ trajectories. The space resolution of the method was estimated to be about 2 cm. The duration of the beam (up to 4 ms) is large enough to overlap the duration of the GDT shots.