The level structure of 172 Dy has been investigated for the first time by means of decay spectroscopy following in-flight fission of a 238 U beam. A long-lived isomeric state with T1/2=0.71(5) s and Kπ=8− has been identified at 1278 keV, which decays to the ground-state and γ -vibrational bands through hindered electromagnetic transitions, as well as to the daughter nucleus 172 Ho via allowed β decays. The robust nature of the Kπ=8− isomer and the ground-state rotational band reveals an axially-symmetric structure for this nucleus. Meanwhile, the γ -vibrational levels have been identified at unusually low excitation energy compared to the neighboring well-deformed nuclei, indicating the significance of the microscopic effect on the non-axial collectivity in this doubly mid-shell region. The underlying mechanism of enhanced γ vibration is discussed in comparison with the deformed Quasiparticle Random-Phase Approximation based on a Skyrme energy-density functional. ; This work was supported by JSPS KAKENHI Grant No. 24740188, 25247045, and 25287065, STFC (UK authors) and the UK National Measurement Office (PHR), the U.S. Department of Energy, Office of Science, Office of Nuclear Physics under Contract No. DE-AC02-06CH11357 (FGK), NRF Korea Grants No. 2009-0093817 and 2013R1A1A2063017 (CSL), 2014S1A2A2028636 and 2016K1A3A7A09005579 (KYC), and Science Foundation Ireland under Grant No. 12/IP/1288 (OJR). The numerical calculations were performed on SR16000 at YITP, Kyoto University, and on COMA (PACS-IX) at CCS, University of Tsukuba.