Aiming at recognizing and localizing objects of novel categories with just a few reference samples, few-shot object detection (FSOD) is quite a challenging task. Previous works rely heavily on the fine-tuning process to transfer their models to the novel categories. They are flawed in the real application since the fine-tuning process is time-consuming and it suffers from serious deterioration on the low-quality support set. Based on the observation, this paper proposes an instant-response and accurate few-shot object detector (IRA-FSOD) that can detect the objects from novel categories without fine-tuning. We carefully analyze the limitations of widely-used Faster R-CNN and transform it to IRA-FSOD. Specifically, we first propose a novel semi-supervised Region Proposal Network (SS-RPN) module and a switch classifier module to precisely recognize the potential foreground instances from novel categories without fine-tuning. Moreover, we introduce two explicit inference strategies into the localization module, including explicit localization score and semi-explicit box regression, to alleviate over-fitting towards the base categories. Extensive experiments demonstrates that the proposed IRA-FSOD not only accomplish few-shot object detection with the instant-response, but also reaches state-of-the-art performance under various FSOD protocols and settings.