The author investigated the influence of pruning on the vegetative growth of Japanese peppermint (Mentha arvensis, L., var. piperascens, MAE.) by pot-culture, and obtained the following results ; 1) The growth of plant-height is temporarily delayed. 2) The third leaf is promoted to develop and as a result the total number of leaves are increased. 3) The time of flowering trends to become more uniform, while is delayed slightly. 4) The irregularity of correlations is seen between the height of plant at the time of treatment and the respective height measured several times there after. 5) The irregularity of correlation is seen between the weight of the subterranean part and weight of the terrestrial part. 6) The dry-weight of leaves and stems are increased. 7) The content of essential oil is increased. 8) The content of menthol in the essential oil is increased.