I rarely have seen this guy in other manga. First time i saw it in The World God Only Know manga, volume 2, chapter 13 and in Kunimitsu manga, chapter 4. Does anyone know where is this guy come from ?
That is a famous advertisement for Oronamin C. The advertise ment is very iconic and most readers would know where it is from, similar to the Uncle Sam ads in the United Stats. In the World Only God Knows, the ad is being parodied, advertising the book instead of the drink.
In Kunimitsu no Matsuri, it is just an advertisement in the background. I am not familiar with the story of Kunimitsu no Matsuri, but I do believe it takes place in relatively recent years, so I'm assuming the ad was placed in the background to show the area is old and dead (because that ad campaign ended in the early 70s).
FYI, the man in the ad is Kon Oomura, a comedian and actor. While most Japanese people know the advertisement, many probably don't know exactly who the man is, since it is such an old ad.
The image you posted is from volume 2, chapter 13 of The World God Only Knows.
This advert too happened in the manga but was advertising the first tankobon volume release of the series, supposedly the old man is Wakaki-sempai himself, otherwise some old Japanese veteran…
I couldn't find a real photograph of Wakaki to compare this drawing, but here are some other self-depictions of him: