Tokyo residents have been left baffled by the appearance of a set of House of Councillors election campaign posters featuring neither the name of a candidate nor the typical headshot, with some asking election officials whether they are illegal or a prank.

The posters were put up on local bulletin boards by a political organization called "Shiji Seito Nashi" (No party to support). The group has fielded four candidates in the Tokyo constituency, and put up four posters each on bulletin boards across Tokyo's 23 wards plus the suburban cities of Machida, Hachioji and Fuchu, among other municipalities.

Voters wondering what the sets of posters are for inundated local electoral committees with inquiries, prompting one senior official to say, "We've never experienced something like this before."

According to the Tokyo Metropolitan Government Election Administration Commission, essentially anything is allowed on a campaign poster under the Public Offices Election Law as long as it does not run counter to public order and morals. It is also not illegal to leave off a candidate's name.

Campaign posters are pasted on bulletin boards in the order of candidacy filings. A drawing takes place only when multiple camps report to the election boards before they officially start accepting candidates on the day the campaign period kicks off, but the first-come, first-serve policy prevails after that.

Because the four "Shiji Seito Nashi" candidates filed their candidacies in a row following a drawing, their posters are displayed side by side. "The arrangement of their posters meets regulations and is not illegal," said an official with the metropolitan election commission.

A Shiji Seito Nashi official told the Mainichi Shimbun, "We are aiming to get people to remember our group name, rather than our candidates' names. We've been doing what we can do within the bounds of the law."

Apart from Tokyo, the group has also fielded one candidate each in the Hokkaido, Kanagawa, Osaka and Kumamoto constituencies, plus another two in the proportional representation bloc. In the 2014 House of Representatives election, the group fielded two candidates in the Hokkaido proportional representation bloc, capturing 104,854 votes.

The upper house election will be held on July 10.