An extended-cavity diode laser operating in the Littrow configuration emitting near 657 nm is stabilized through its injection current to a reference cavity with a finesse of more than 105 and a corresponding resonance linewidth of 14 kHz. The laser linewidth is reduced from a few megahertz to a value below 30 Hz. The compact and robust setup appears ideal as a portable optical frequency standard that uses the calcium intercombination line.