The invention discloses a selective amplifying device. Lasers with specific modes are selected from multi-mode lasers and amplified. The multi-mode lasers (8A) and excitation light (34A) for amplifying are inputted to a gain medium (62) for amplifying according to the relationship of optical axis consistency of the multi-mode lasers and the excitation light, and the effective beam diameter of theexcitation light for amplifying is smaller than that of the lasers with the specific modes in the gain medium (62). Results show that the lasers lower than the specific modes within an irradiation range of the excitation light (34A) for amplifying are selectively amplified, and the lasers (40A) with cleaned modes are outputted.