The present invention relates to an antimicrobial peptide isolated from Halocynthia aurantium, more particularly, to an antimicrobial peptide isolated from the body fluid of Halocynthia aurantium and an antimicrobial agent comprising the same as an active ingredient. The antimicrobial peptide of the present invention shows excellent antimicrobial activity under strong acidic and basic environments. Moreover, it also shows strong antimicrobial activity against resistant bacteria. So, it can be used usefully as a natural antimicrobial agent.