A retaining unit includes: a movable ejector pin; a ring-shaped member used in a fixed state; and a ball plunger directly attached to the ejector pin or fixed to a fixing member attached to the ejector pin. The ball plunger is configured to press the ring-shaped member and engage the ejector pin with the ring-shaped member. When a force at a predetermined level or higher is applied to the ejector pin in a movement direction of the ejector pin, the engagement with the ring-shaped member is released so that the ejector pin is allowed to move. The ball plunger is incorporated into the ejector pin or the fixing member so as to be unitized together.