The guard of Pananagar MLA Narendra Tripathi identified as Hanuman Prasad reached well inside the building by crossing check gates including the one where metal detectors were installed.
The security personnel at the assembly, however, noticed him and took possession of his pistol. Later, he was asked to leave the assembly premises.
State Home Minister Uma Shankar Gupta said the matter was not a breach of security though it was being looked into.
Speaker Ishwardas Rohani was also briefed about the incident, sources said.