Abstract: A blow-by gas treatment device for an engine configured such that the upper end of a portion of a passage for introducing fresh air and discharging blow-by gas vertically faces an opening section, the portion being located further toward the crank chamber side than a first breather chamber, and also such that a communication section communicating with the space within a variable valve chamber is provided between the upper end and the opening section. The area of the communication of the communication section is set so that the amount of increase in the pressure within the variable valve chamber relative to the pressure within the first breather chamber does not exceed a predetermined value when blow-by gas flows to the air intake passage not only from a passage dedicated for discharging blow-by gas but also from the passage for introducing fresh air and discharging blow-by gas.