A magnetic tape cassette of the .beta. cam or .beta. cam SP type having an improved mistake erasing preventing construction and improved dustproofing property. For the .beta. cam SP type cassette, a movable plug is provided in an insertion hole formed in the bottom face of the cassette housing and a snap-off tab is removed beforehand, while for the .beta. cam type cassette, an immovable plug is provided in the insertion hole and the snap-off tab is left intact.