Synuchus cycloderus is a ground beetle species commonly found in woodlands over a wide range in East Asia. We found a remarkable increase in the population density of this species after logging or vegetation cutting in forests. This sensitivity to the environmental changes potentially enable us to use this species as a bioindicator. However, feeding habits and other important ecological traits of this species are little known. Here we study feeding habits of S. cycloderus by means of gut dissection method. We analyzed 112 out of 1,273 individuals collected by pitfall traps during the spring (May to July) and autumn (Oct. to Dec.) in 2012 at a Satoyama Oaota-no-mori forest (100 ha, 35°54’ N, 139°55’ E, 18 to 25 m a.s.l., Chiba Prefecture). Gut contents were examined using a microscope. We detected fragments of prey in 83 guts (74%), the others were empty or contained fluid/amorphous materials. Synuchus cycloderus seemed to be a generalist predator that preys voraciously and successively on various types of small arthropods often consuming them without chewing. In the guts we also found compound eyes and feet with a pair of claws suggesting that the beetle can hunt not only larvae but also adults (of probably flies) as prey. Collembola species including Hypogastrura gracilis were detected in 11 guts. Overall, whole or identifiable fragments of prey were detectable in S. cycloderus guts indicating that gut dissection was an effective method for determining the diets of this species.