In a head protecting airbag device, an airbag includes an inflation portion and a connection port portion. The connection port portion includes: an outlet side portion having a communication port to the inflation portion at its lower end; and an inlet side portion for inserting an inflator therein. The inflator includes a body portion having a gas discharge port, and a diffuser having a gas outlet port. The diffuser is connected with the body portion so as to cover the gas discharge port, having its two ends forced into contact with the outer circumference of the body portion. The inflator is connected to a connection port portion with good sealing properties and the gas outlet port faces toward the communication port of the connection port portion and thus toward the inflation portion. This head protecting airbag device simplifies the connection structure between the inflator and the airbag while retaining the quick expansion and inflation of the airbag.