A driving transmission device includes a storage part, a fixing pin and a supporting part. The storage part has a through hole. The fixing pin rotatably supports a gear. To the supporting part, the fixing pin is fixed. A hole side chamfered portion is formed at an outside opening edge of the through hole. A shaft side chamfered portion is formed at the tip end portion of the fixing pin. A lubricant is applied to a contact area between the fixing pin and the through hole. A hole side boundary portion between an inner circumferential surface of the through hole and the hole side chamfered portion is in contact with an outer circumferential surface of the fixing pin. A shaft side boundary portion between an outer circumferential surface of the fixing pin and the shaft side chamfered portion faces the hole side chamfered portion with a gap.