To provide a small-scale and low-cost security system, which detects and intimates an unpermitted unauthenticated entrance in a multiple dwelling house such as a condominium, when the passing person number of residents or visitors permitted by a resident is differed from the number of persons who actually entered in the condominium.

The security system for multiple dwelling house adapted to permit entrance/exit of specified persons at a common entrance comprises a resident authentication means for authenticating that a passing person is an inhabitant of the multiple dwelling house; a door control means for an automatic door which is unlock-operated through the authentication means; and a passing person detection means for detecting the direction and number of persons passing through the automatic door. When authentication by the authentication means is successful, unlocking of the door is instructed to the door control means, and detection start for the number of persons who pass through the automatic door is instructed to the passing person detection means. When the passing person detection means detects entering of persons of a number exceeding N into the multiple dwelling house through the door in spite of that the frequency of unlock instruction to the authentication means is N, an alarm means gives an alarm for abnormality occurrence.

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