Multiple outbursts of a Jupiter-family comet (JFC), 15P/Finlay, occurred from late 2014 to early 2015. We conducted an observation of the comet after the first outburst and subsequently witnessed another outburst on 2015 January 15.6-15.7. The gas, consisting mostly of C2 and CN, and dust particles expanded at speeds of 1110 ± 180 m s-1 and 570 ± 40 m s-1 at a heliocentric distance of 1.0 au. We estimated the maximum ratio of solar radiation pressure with respect to the solar gravity β max = 1.6 ± 0.2, which is consistent with porous dust particles composed of silicates and organics. We found that 108-109 kg of dust particles (assumed to be 0.3 μm-1 mm) were ejected through each outburst. Although the total mass is three orders of magnitude smaller than that of the 17P/Holmes event observed in 2007, the kinetic energy per unit mass (104 J kg-1) is equivalent to the estimated values of 17P/Holmes and 332P/2010 V1 (Ikeya-Murakami), suggesting that the outbursts were caused by a similar physical mechanism. From a survey of cometary outbursts on the basis of voluntary reports, we conjecture that 15P/Finlay-class outbursts occur 〉1.5 times annually and inject dust particles from JFCs and Encke-type comets into interplanetary space at a rate of ̃10 kg s-1 or more.
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