A holding device has an elastic locking piece having an extending part extending to the outside from a locking claw part. The contact part of an unlocking operating piece contacts the outside of the extending part to reduce the unlocking force of the unlocking operating piece. A sawtooth-form part is in a locked state when a locking piece of an elastic locking piece enters the sawtooth-form part. The elastic locking piece has the extending part extending outside from the locking claw part. The unlocking operating piece has a contact part for contacting with the extending part. The contact part contacts the outside of the extending part when the unlocking operating piece is pressed, the locking claw part of the elastic locking piece is ejected from the indented part of the sawtooth-form part by the contact, and the locked state between the sawtooth-form part and the locking claw part is unlocked.