Magnitude–color diagram of ADF22.A2 where the horizontal axis shows Ks band magnitude in the AB system, while the vertical axis shows the flux ratio S_1.2 mm/S_3.6 μm in a logarithmic scale. We also show HST-dark galaxies found by ALMA deep surveys (Franco et al. 2018; Yamaguchi et al. 2019) and K-blank SMGs with z_spec (Swinbank et al. 2012; Wang et al. 2009; Pavesi et al. 2018). It is important to note that sources that do not have a reliable photometry due to source confusion were excluded. For comparison purposes, K-blank SMGs from Simpson et al. (2014) and K-band detected SMGs in the ALMA SCUBA-2 UDS survey (AS2UDS, Dudzevičiūtė et al. 2020) are also displayed. We adopted 3σ upper limits for K-blank sources. ADF22.A2 shares a similar region with other near-infrared-dark ALMA sources, which demonstrates that they belong to an equivalent population. The z_phot distribution of SMGs in AS2UDS and some z_spec for the near-infrared-dark population suggests that the population is generally estimated to be at z >  3.5 and ADF22.A2 is one example of them.