A connector that can be connected to a first container and a second container to transfer a liquid between the first container and the second container, the connector comprising: a first connecting portion that is engageable with the first container and a second connecting portion that is engageable with the second container, wherein the second connecting portion is integrally provided with an engagement portion that is engageable with the second container and a needle-like portion for piercing into the second container, the connector comprising: a connector main body provided with a tubular portion and a stopcock that is fitted in the tubular portion so as to be rotatable around an axis of the tubular portion, characterized in that one end of the tubular portion is sealed with the stopcock that is fitted in the tubular portion, an open portion is formed at the other end of the tubular portion, a penetrating flow channel that passes through the stopcock in a radial direction thereof is formed in the stopcock, a first hole, a second hole, a third hole, and a fourth hole are formed in the connector main body, the first, the second, the third, and the fourth holes are holes that bring an inner space of the tubular portion into communication with an external space of the connector main body, a flow channel that brings an opening formed on the open portion side into communication with an opening formed in a side face of the stopcock is formed in the stopcock, and switching between a setting that brings the first hole into communication with the flow channel in the stopcock and brings the third hole into communication with the fourth hole via the penetrating flow channel and a setting that brings the second hole into communication with the flow channel in the stopcock and brings the third hole into communication with the fourth hole via the penetrating flow channel can be achieved by rotating the stopcock. a first connecting portion that is engageable with the first container and a second connecting portion that is engageable with the second container, wherein the second connecting portion is integrally provided with an engagement portion that is engageable with the second container and a needle-like portion for piercing into the second container, the connector comprising: a connector main body provided with a tubular portion and a stopcock that is fitted in the tubular portion so as to be rotatable around an axis of the tubular portion, characterized in that one end of the tubular portion is sealed with the stopcock that is fitted in the tubular portion, an open portion is formed at the other end of the tubular portion, a penetrating flow channel that passes through the stopcock in a radial direction thereof is formed in the stopcock, a first hole, a second hole, a third hole, and a fourth hole are formed in the connector main body, the first, the second, the third, and the fourth holes are holes that bring an inner space of the tubular portion into communication with an external space of the connector main body, a flow channel that brings an opening formed on the open portion side into communication with an opening formed in a side face of the stopcock is formed in the stopcock, and switching between a setting that brings the first hole into communication with the flow channel in the stopcock and brings the third hole into communication with the fourth hole via the penetrating flow channel and a setting that brings the second hole into communication with the flow channel in the stopcock and brings the third hole into communication with the fourth hole via the penetrating flow channel can be achieved by rotating the stopcock. a connector main body provided with a tubular portion and a stopcock that is fitted in the tubular portion so as to be rotatable around an axis of the tubular portion, characterized in that one end of the tubular portion is sealed with the stopcock that is fitted in the tubular portion, an open portion is formed at the other end of the tubular portion, a penetrating flow channel that passes through the stopcock in a radial direction thereof is formed in the stopcock, a first hole, a second hole, a third hole, and a fourth hole are formed in the connector main body, the first, the second, the third, and the fourth holes are holes that bring an inner space of the tubular portion into communication with an external space of the connector main body, a flow channel that brings an opening formed on the open portion side into communication with an opening formed in a side face of the stopcock is formed in the stopcock, and switching between a setting that brings the first hole into communication with the flow channel in the stopcock and brings the third hole into communication with the fourth hole via the penetrating flow channel and a setting that brings the second hole into communication with the flow channel in the stopcock and brings the third hole into communication with the fourth hole via the penetrating flow channel can be achieved by rotating the stopcock.