a)F_disk/F_∗ at AKARI 18 μm versus stellar age for F-type stars. Filled circles indicate debris disk samples from previous works at 24 μm by Spitzer/MIPS (Beichman et al. 2005, 2006; Bryden et al. 2006; Chen et al. 2005a,b; Hillenbrand et al. 2008; Trilling et al. 2008). Filled squares represent excess ratios observed by AKARI at 18 μm for stars with excesses larger than 3σ (debris disk candidates), while open squares show that for all stars in our sample. The solid line indicates the evolutionary track with t₀ = 0.5 Gyr, where t₀ is the dissipation time scale, while the dotted line indicates evolutionary track of t₀ = 2 Gyr (see text for details). b) Same as a) but for G-type stars.