Can police ask questions without warrant?

I had an argument with a so-so acquaintance of mine in which he started arguing that in Japan you need a warrant so that the police can interview people connected to the case under investigation. Specifically, the issue was about a missing child, and I was saying that the police should ask the relatives in case they knew something important. My acquaintance said that the police had no right to question relatives without a warrant. Is this true or is my acquaintance talking nonsense?

I've seen this written about before - all they need is a suspicion that you've done something. That could be as simple as thinking you have a knife in your bag (contrary to the Weapons Control Act), or you have some illegal substance. I've seen lots of occasions where police get ID, question and search people - none of those would have involved a warrant.
This isn't the USA where different rules apply regarding rights, questioning and "reasonable articulatable suspicion".

On the street,

Police can ask you question, but it is up to you to answer or just ignore them. HOWEVER they will press on with their question provocation until you somehow get irritated/lost control and do something stupid like touching them. It's one of their tactic to further 'fishing' for information.

First you'll see one or two officer come and questioning you, friendly, and if you ignore them, and they will escalate it, few more will come surround you, try to intimidate you, and in order for you to make mistake such like pushing them away, which the give them reason to 'act'.

This then, they will have reasoning to either detain you, bring you to police station for more details questioning/interrogation. That they can do without any charges/warrant, can hold you up to 23days.

@..:
This then, they will have reasoning to either detain you, bring you to police station for more details questioning/interrogation. That they can do without any charges/warrant, can hold you up to 23days.

That's not correct.

To detain a suspect before prosecution,
a prosecutor should request a detention warrant
within 72 hours from the time the suspect is placed under physical restraint.

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When police officers suspect you to be related to a crime,
they may question you without a warrant,
in which case you can refuse to answer.
Such questioning by the police is called 職務質問 [shokumu-shitsumon].