A handler may include a handler system main body used for testing semiconductor devices; an open-type stocker portion on a front side of the handler system main body; and/or a plurality of single-door-type stockers in the open-type stocker portion. The single-door-type stockers may include windows on upper parts of front sides of the single-door-type stockers. The front sides of the single-door-type stockers may be protected. The handler also may include a front top door on an upper part of the front side of the handler system main body; locking stoppers below the windows of the single-door-type stockers; safety sensors on sides of the open-type stocker portion; and/or a working table in front of the open-type stocker portion. The open-type stocker portion may be below the front top door. The safety sensors may stop the handler when the single-door-type stockers are not closed.