A substrate processing device includes a depressurizable hot wall chamber having a sidewall with a temperature which becomes higher than room temperature and a first substrate transferring port provided in the sidewall, a depressurizable transfer chamber having a transfer arm mechanism and a second substrate transferring port, and a gate valve unit provided between the hot wall chamber and the transfer chamber. The gate valve unit includes: a housing having a sidewall provided with communicating holes, a first housing substrate transferring port, and a second housing substrate transferring port; a valve body which is elevatable in the housing; and a double sealing structure having a first sealing member and a second sealing member provided at an outer side of the first sealing member. The communicating holes communicate a gap between the first sealing member and the second sealing member with an internal space of the housing.