What's more, from what I can remember, Ash's father hasn't really been referenced in the anime, save for a mention or two in the first season. He certainly hasn't appeared.
For years now, there's been speculation about the character's father, such as that he's dead à la Goku's dad, or that one of the older male Pokémon characters is his pops, or that Ash doesn't even have a father. That, you know, he's was immaculately conceived. Like Jesus.
Ash not having a father—and that father never really being addressed—does give the anime subtext that provides endless conjuncture.
But what's the official line on Ash's father? This magazine scan, which is from a while back but is once again appearing online in Japan, seems to shed some light on Ash's dad.
In the magazine, Ash is asked where his dad is. Ash replies, "Because he said he was going to become a Pokémon Trainer, he left on a journey and hasn't come back home since. Eventually, our paths might cross!"
This jives with what the original director of the Pokémon anime, Masamitsu Hidaka, said back in 2008. Hidaka also told website Pokebeach that at Ash's grandfather was also a great Pokémon Trainer, which is why his father became one, too.
At that time, Hidaka also said that Ash's father might eventually appear, especially if the show's producers felt that the character needed to "grow". Certainly would make for a weepy yarn if they chose to go that route.