Blow-by gas treatment device

This blow-by gas treatment device, which is mounted in an internal combustion engine of a vehicle, is provided with: a blow-by gas pipe (10) having a downward outlet section (9) open to the atmosphere; and a collection container (20) into which the outlet part of the blow-by gas pipe is inserted and which collects the oil discharged from the outlet part of the blow-by gas pipe. The collection container is provided with: an upper end opening (22) which is formed in the upper end (21) of the collection container and into which the outlet part of the blow-by gas pipe is inserted from above; a discharge hole (24) formed in an upper portion of a rear surface portion (23) of the collection container, the rear surface portion (23) being located rearward in the vehicle longitudinal direction; and a cutout section (26) formed in the upper part of a side surface section (25) of the collection container, the side surface section (25) being located on the outside in the vehicle width direction. 被搭载于车辆的内燃机的窜缸混合气处理装置包括：窜缸混