Animal parents provide various types of care for their eggs during the incubation period. Here, we discovered the egg-rolling behaviour in a subsocial burrower bug, Adomerus rotundus with egg-mass guarding on the ground. Because there is a thermal gradient between ground surface and air in their habitat, the lower and upper eggs in an egg-mass are exposed to different thermal levels. To examine whether the bug’s egg-rolling behaviour mitigates the uneven thermal environment, we observed the developmental stage of individual eggs in an egg-mass at uniform and non-uniform thermal conditions. The embryonic development was synchronous under each condition in the control group, which experienced continuous egg-rolling by a mother, but was not in the isolated group under the non-uniform condition. However, when we artificially rolled an egg-mass under the non-uniform condition, the embryonic development was synchronous even in the isolated group. These results strongly suggest that A. rotundus mothers actively regulate thermal microenvironment of the eggs by the egg-rolling behaviour. Although thermal regulation of eggs is known to be a highly developed parental care evolved in vertebrates, this is the first evidence of thermal egg care in an invertebrate.