A substrate storage container includes a locking mechanism, provided in a lid for opening and closing an opened front portion of a container body which stores a substrate of three sheets or less, for locking the lid fitted into the front portion of the container body. The locking mechanism is supported by the lid which is substantially laterally long when viewed from the front. The locking mechanism includes an advance/retreat engaging body that opposes an inner periphery of the front portion of the container body when the lid is fitted into the front portion of the container body, and a spring member that causes the advance/retreat engaging body to advance from the lid to the inner periphery of the front portion of the container body and thus causes a tip end portion side thereof to interfere when the lid is fitted into the front portion of the container body.